Ab Initio Counterparts Research Articles

Overtone spectroscopy of v(C=O) stretching vibration of hexafluoroacetone: Experimental and ab initio determination of peak positions, absolute intensities, and band shapes

Infrared overtone spectra of the ν(C = O) stretching vibration (ν1) of gaseous hexafluoroacetone ((CF3)2C = O, HFA) were recorded in the spectral range of 7450-3300 cm-1 with a resolution of 0.1 cm-1. Experimental absolute IR intensities and vibrational band centers of the overtones 2ν1, 3ν1, 4ν1 of HFA were measured and compared with their ab initio counterparts, calculated by the second-order canonical vibrational perturbation theory (CVPT2). A hybrid potential energy surface (PES) was evaluated using the quantum-mechanical models MP2/cc-pVQZ for harmonic and MP2/cc-pVTZ for anharmonic parts. Cubic surfaces of dipole moment components were determined using the MP2/cc-pVTZ model. The predicted IR intensities for the first and second overtones reproduced the experimental values with a discrepancy of 6% and 9%, respectively. A weak Fermi resonance between the first overtone 2ν1 and combination tone ν1 + ν2 + ν8 was predicted. The appropriate model was employed for simulating the bands studied using: (i) the asymmetric-top vibration-rotational structure of the ν1 mode, (ii) the inhomogeneous band structure due to contributions of the vibrational states populated at the room temperature, and (iii) the homogeneous broadening of each vibration-rotation transition due to the intramolecular vibrational redistribution (IVR). The rotational and vibrational anharmonic constants were taken from the ab initio calculations, whereas the IVR data were obtained from the experimental data of Chekalin et al. (JPCA 2014, 118:955) with a time resolution of ≈100 fs. A good level of agreement of the predicted shapes of bands studied with experiment is achieved.

Role of Augmented Basis Sets and Quest for ab Initio Performance/Cost Alternative to Kohn-Sham Density Functional Theory.

Following a previous work, we have assessed the feasibility of MP2/CBS(d, t) as an alternative to state-of-the-art density functionals. The effect of using augmented basis sets is here tested on the 76 barrier heights and 10 isomerization reactions previously utilized. Moreover, calculations for 20 sets of the GMTKN24 database for thermochemistry, kinetics, and noncovalent interactions have been performed. For the density functional theory calculations, M06-2X and B3LYP-D3 functionals are utilized as two representative functionals, while MP2 and CCSD(T) methods are employed as the ab initio counterparts. The results show that MP2 calculations perform similarly to the ones obtained with M06-2X insofar as accuracy and computational cost are concerned. For all methods, the use of augmented basis sets yields enhanced results for anionic systems when compared with the ones from non-augmented bases. Otherwise, the basis-set change effect is found to be minimal. It is therefore concluded that the use of large basis sets is unjustified when facing the increase in computational cost.

Structure and Stability of Molecular Crystals with Many-Body Dispersion-Inclusive Density Functional Tight Binding.

Accurate prediction of structure and stability of molecular crystals is crucial in materials science and requires reliable modeling of long-range dispersion interactions. Semiempirical electronic structure methods are computationally more efficient than their ab initio counterparts, allowing structure sampling with significant speedups. We combine the Tkatchenko-Scheffler van der Waals method (TS) and the many-body dispersion method (MBD) with third-order density functional tight-binding (DFTB3) via a charge population-based method. We find an overall good performance for the X23 benchmark database of molecular crystals, despite an underestimation of crystal volume that can be traced to the DFTB parametrization. We achieve accurate lattice energy predictions with DFT+MBD energetics on top of vdW-inclusive DFTB3 structures, resulting in a speedup of up to 3000 times compared with a full DFT treatment. This suggests that vdW-inclusive DFTB3 can serve as a viable structural prescreening tool in crystal structure prediction.

Long-range behavior of the transition dipole moments of heteronuclear dimers XY (X, Y = Li, Na, K, Rb) based on ab initio calculations.

The ab initio electronic transition dipole moments (ETDMs) of heteronuclear dimers XY (X, Y = Li, Na, K, Rb) were calculated between the ground and excited states converging to the lowest three dissociation limits. The spin-allowed ETDMs were evaluated in a wide range of interatomic distances, R, by means of the quasi-relativistic electronic wave functions obtained by the multi-reference configuration interaction method. The inner-shell electrons (2 electrons for Li and Na atoms, and 10 and 28 for K and Rb, respectively) were described using the non-empirical shape-consistent effective core potentials. The l-independent core polarization potentials of each atom were used to take core-polarization and core-valence correlation effects into account. The long-range behavior of both singlet-singlet X1Σ+-(2,3)1Σ+;(1,2)1Π and triplet-triplet a3Σ+-(2,3)3Σ+;(1,2)3Π transition moments is perfectly fitted at large R-distance by the asymptotic formula of X. Chu and A. Dalgarno, Phys. Rev. A: At., Mol., Opt. Phys., 2002, 66, 024701: , where the coefficient β is equal to 2 and -1 for the Σ-Σ and Σ-Π transitions, respectively. The n2S-n2P transition moments, dA, and dynamic polarizabilites, αB, of the alkali atoms in the n2S state extracted from the present molecular calculations coincide with their empirical and ab initio counterparts to within a few percent.

Identification and Characterization of Molecular Bonding Structures by ab initio Quasi-Atomic Orbital Analyses.

The quasi-atomic analysis of ab initio electronic wave functions in full valence spaces, which was developed in preceding papers, yields oriented quasi-atomic orbitals in terms of which the ab initio molecular wave function and energy can be expressed. These oriented quasi-atomic orbitals are the rigorous ab initio counterparts to the conceptual bond forming atomic hybrid orbitals of qualitative chemical reasoning. In the present work, the quasi-atomic orbitals are identified as bonding orbitals, lone pair orbitals, radical orbitals, vacant orbitals and orbitals with intermediate character. A program determines the bonding characteristics of all quasi-atomic orbitals in a molecule on the basis of their occupations, bond orders, kinetic bond orders, hybridizations and local symmetries. These data are collected in a record and provide the information for a comprehensive understanding of the synergism that generates the bonding structure that holds the molecule together. Applications to a series of molecules exhibit the complete bonding structures that are embedded in their ab initio wave functions. For the strong bonds in a molecule, the quasi-atomic orbitals provide quantitative ab initio amplifications of the Lewis dot symbols. Beyond characterizing strong bonds, the quasi-atomic analysis also yields an understanding of the weak interactions, such as vicinal, hyperconjugative and radical stabilizations, which can make substantial contributions to the molecular bonding structure.

Fourier Transform Microwave Spectrum of Propene-3-d1 (CH2═CHCH2D), Quadrupole Coupling Constants of Deuterium, and a Semiexperimental Equilibrium Structure of Propene.

The ground-state rotational spectrum of propene-3-d1, CH2═CHCH2D, was measured by Fourier transform microwave spectroscopy. Transitions were assigned for the two conformers, one with the D atom in the symmetry plane (S) and the other with the D atom out of the plane (A). The energy difference between the two conformers was calculated to be 6.5 cm-1, the S conformer having lower energy. The quadrupole hyperfine structure due to deuterium was resolved and analyzed for both conformers. The experimental quadrupole coupling and the centrifugal distortion constants compared favorably to their ab initio counterparts. Ground-state rotational constants for the S conformer are 40582.157(9), 9067.024(1), and 7766.0165(12) MHz. Ground-state rotational constants for the A conformer are 43403.75(3), 8658.961(2), and 7718.247(2) MHz. For the A conformer, a small tunneling splitting (19 MHz) due to internal rotation was observed and analyzed. Using the new rotational constants of this work as well as those previously determined for the 13C species and for some deuterium-substituted species from the literature, a new semiexperimental equilibrium structure was determined and its high accuracy was confirmed. The difficulty in obtaining accurate coordinates for the out-of-plane hydrogen atom is discussed.

Experimental and theoretical studies of the coupledA1Σ+andbΠ3states of NaK

We report an extensive series of transitions (including collisional transfer lines) from pure and mixed levels of the NaK ${A}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}$ and $b\phantom{\rule{0.16em}{0ex}}^{3}\mathrm{\ensuremath{\Pi}}$ states to the ${X}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}$ state, observed at the Universit\'e Lyon 1 using Fourier-transform spectroscopy. We then combine these data with previously reported data on these states from emission from the ${B}^{1}\mathrm{\ensuremath{\Pi}}$ and ${C}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}$ states and from mutually perturbed levels of the $D{\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}$ and $d\phantom{\rule{0.16em}{0ex}}^{3}\mathrm{\ensuremath{\Pi}}$ states. We obtain 2758 distinct term values: the full data set includes 11 624 term values, with many multiple determinations from transitions over a range of vibrational and rotational levels. The data are analyzed by fitting to potentials of the ``Hannover'' form [C. Samuelis et al., Phys. Rev. A 63, 012710 (2000)] plus spin-orbit (SO) functions in a simple Morse form, yielding an rms residual of approximately 0.029 ${\mathrm{cm}}^{\ensuremath{-}1}$. The empirical SO functions agree well with their ab initio counterparts obtained from electronic structure calculations based on nonempirical effective core potentials. From level energies of the $A\ensuremath{-}b$ complex calculated from the fitted potentials and SO functions, we identify reasonable candidates for transitions between Feshbach resonance states and mixed singlet-triplet gateway levels of the ${A}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}\ensuremath{-}b\phantom{\rule{0.16em}{0ex}}^{3}\mathrm{\ensuremath{\Pi}}$ manifold, leading either to $v=0$ levels of the $X$ state or to mixed singlet-triplet levels at higher energies that can be used for perturbation-facilitated double-resonance experiments.

Why it is sometimes difficult to determine the accurate position of a hydrogen atom by the semiexperimental method: structure of molecules containing the OH or the CH3 group.

The semiexperimental (SE) technique, whereby equilibrium rotational constants are derived from experimental ground-state rotational constants and corrections based on an ab initio cubic force field, has the reputation to be one of the most accurate methods to determine an equilibrium structure ( reSE). However, in some cases, it cannot determine accurately the position of the hydrogen. To investigate the origins of this difficulty, the SE structures of several molecules containing either the OH or the CH3 group are determined and compared to their best ab initio counterparts. It appears that an important factor is the accuracy of the geometry used to calculate the force field, in particular when the least-squares system is not well conditioned. In this case, the mixed regression method is often an easy way to circumvent this difficulty.

Automated Repulsive Parametrization for the DFTB Method.

The density-functional-based tight-binding method is an efficient scheme for quantum mechanical atomistic simulations. While the most relevant part of the chemical energies is calculated within a DFT-like scheme, a fitted correction function-the repulsive energy-is used to achieve results as close to ab initio counterparts as possible. We have developed an automatic parametrization scheme to ease the process of the repulsive energy fitting, offering a more systematic and much faster alternative to the traditional fitting process. The quality of the resulting repulsives can be tuned by selecting and weighting the fit systems and the important physical properties (energy, force, Hessian) of them. Besides driving DFT calculators in the fitting process automatically, the flexibility of our scheme also allows the usage of external data (e.g., molecular dynamics trajectories or experimental data) as a reference. Results with several elements show that our procedure is able to produce parameter sets comparable to handmade ones, yet requiring far less human effort and time.

Beyond the Twilight Zone: Automated prediction of structural properties of proteins by recursive neural networks and remote homology information

AbstractThe prediction of 1D structural properties of proteins is an important step toward the prediction of protein structure and function, not only in the ab initio case but also when homology information to known structures is available. Despite this the vast majority of 1D predictors do not incorporate homology information into the prediction process. We develop a novel structural alignment method, SAMD, which we use to build alignments of putative remote homologues that we compress into templates of structural frequency profiles. We use these templates as additional input to ensembles of recursive neural networks, which we specialise for the prediction of query sequences that show only remote homology to any Protein Data Bank structure. We predict four 1D structural properties – secondary structure, relative solvent accessibility, backbone structural motifs, and contact density. Secondary structure prediction accuracy, tested by five‐fold cross‐validation on a large set of proteins allowing less than 25% sequence identity between training and test set and query sequences and templates, exceeds 82%, outperforming its ab initio counterpart, other state‐of‐the‐art secondary structure predictors (Jpred 3 and PSIPRED) and two other systems based on PSI‐BLAST and COMPASS templates. We show that structural information from homologues improves prediction accuracy well beyond the Twilight Zone of sequence similarity, even below 5% sequence identity, for all four structural properties. Significant improvement over the extraction of structural information directly from PDB templates suggests that the combination of sequence and template information is more informative than templates alone. Proteins 2009. © 2009 Wiley‐Liss, Inc.

Long-range multipolar potentials of the 18 spin-orbit states arising from the C(P3)+OH(X Π2) interaction

We present multipolar potentials at large intermolecular distances for the 18 doubly degenerate spin-orbit states arising from the interaction between the two open-shell systems, C((3)P) and OH(X (2)Pi). With OH fixed at its ground vibrational state-averaged distance r(0), the long-range potentials are two-dimensional potential energy surfaces (PESs) that depend on the intermolecular distance R and the angle gamma = CGH, where G represents the mass center of OH. The 18x18 diabatic potential matrix elements are built up from the perturbation theory up to second order and from a two-center expansion of the Coulombic interaction potential, resulting in a multipolar expansion of the potential expressed as a series of terms varying in R(-n). The expressions for the long-range coefficients of the expansion are explicitly given in terms of monomer properties such as permanent multipole moments, and static and dynamic polarizabilities. Accurate values for the monomer properties are used to properly determine the long-range interaction coefficients. The diagonalization of the full 18x18 potential matrix generates adiabatic long-range PESs in good agreement with their ab initio counterparts.

Impact-parameter dependence of the energy loss of fast molecular clusters in hydrogen

The electronic energy loss of molecular clusters as a function of impact parameter is far less understood than atomic energy losses. For instance, there are no analytical expressions for the energy loss as a function of impact parameter for cluster ions. In this work, we describe two procedures to evaluate the combined energy loss of molecules: Ab initio calculations within the semiclassical approximation and the coupled-channels method using atomic orbitals; and simplified models for the electronic cluster energy loss as a function of the impact parameter, namely the molecular perturbative convolution approximation (MPCA, an extension of the corresponding atomic model PCA) and the molecular unitary convolution approximation (MUCA, a molecular extension of the previous unitary convolution approximation UCA). In this work, an improved ansatz for MPCA is proposed, extending its validity for very compact clusters. For the simplified models, the physical inputs are the oscillators strengths of the target atoms and the target-electron density. The results from these models applied to an atomic hydrogen target yield remarkable agreement with their corresponding ab initio counterparts for different angles between cluster axis and velocity direction at specific energies of 150 and 300 keV/u.

Spectroscopic signatures of nonrigidity: Algebraic analyses of infrared and Raman transitions in nonrigid species

An algebraic scheme is proposed to explore the influence of nonrigidity upon spectroscopic (infrared/Raman) transitions. Application of this approach to large amplitude bending motion relies upon a Hamiltonian that embodies both rigidly-linear and rigidly-bent behavior, with the attendant U(3) dynamical algebra uniformly describing all pertinent operators. The bending mode supported by the quasilinear carbon trimer (C 3) is investigated, yielding algebraic predictions for vibrational energy level patterns and infrared transition matrix elements that are in reasonable accord with their ab initio counterparts. A coherent state formalism is employed to extract dependence of potential energy and dipole moment functions on C 3 bending coordinate.

Representation of Zn(II) complexes in polarizable molecular mechanics. Further refinements of the electrostatic and short‐range contributions. Comparisons with parallel ab initio computations

We present refinements of the SIBFA molecular mechanics procedure to represent the intermolecular interaction energies of Zn(II). The two first-order contributions, electrostatic (E(MTP)), and short-range repulsion (E(rep)), are refined following the recent developments due to Piquemal et al. (Piquemal et al. J Phys Chem A 2003, 107, 9800; and Piquemal et al., submitted). Thus, E(MTP) is augmented with a penetration component, E(pen), which accounts for the effects of reduction in electronic density of a given molecular fragment sensed by another interacting fragment upon mutual overlap. E(pen) is fit in a limited number of selected Zn(II)-mono-ligated complexes so that the sum of E(MTP) and E(pen) reproduces the Coulomb contribution E(c) from an ab initio Hartree-Fock energy decomposition procedure. Denoting by S, the overlap matrix between localized orbitals on the interacting monomers, and by R, the distance between their centroids, E(rep) is expressed by a S(2)/R term now augmented with an S(2)/R(2) one. It is calibrated in selected monoligated Zn(II) complexes to fit the corresponding exchange repulsion E(exch) from ab initio energy decomposition, and no longer as previously the difference between (E(c) + E(exch)) and E(MTP). Along with the reformulation of the first-order contributions, a limited recalibration of the second-order contributions was carried out. As in our original formulation (Gresh, J Comput Chem 1995, 16, 856), the Zn(II) parameters for each energy contribution were calibrated to reproduce the radial behavior of its ab initio HF counterpart in monoligated complexes with N, O, and S ligands. The SIBFA procedure was subsequently validated by comparisons with parallel ab initio computations on several Zn(II) polyligated complexes, including binuclear Zn(II) complexes as in models for the Gal4 and beta-lactamase metalloproteins. The largest relative error with respect to the RVS computations is 3%, and the ordering in relative energies of competing structures reproduced even though the absolute numerical values of the ab initio interaction energies can be as large as 1220 kcal/mol. A term-to-term identification of the SIBFA contributions to their ab initio counterparts remained possible even for the largest sized complexes.

Modeling of Copper(II) Complexes with the SIBFA Polarizable Molecular Mechanics Procedure. Application to a New Class of HIV-1 Protease Inhibitors

We have previously developed a new family of organometallic complexes targeting the HIV-1 protease, an enzyme that is essential for viral maturation. Among these, two Cu2+ complexes C1 and C2 were synthesized from flexible ligands L1, N1-(4-methyl-2-pyridyl)-2,3,6-trimethoxybenzamide and L2, N2-(2-methoxybenzyl)-2-quinolinecarboxamide, respectively. These ligands, designed to fit the protease active site, were shown to form 2:1 complexes with Cu2+. To compare the relative stability of C1 and C2 and to study the energetic and structural aspects of such large Cu2+ complexes, we have extended the polarizable molecular mechanics procedure SIBFA to treat this cation. This was done by carrying out parallel ab initio (HF, MP2) as well as DFT computations. A first validation step was done on monoligated complexes, in which the SIBFA energy components were shown to correctly match their ab initio counterparts from an energy-decomposition procedure. Subsequent tests on polyligated Cu2+ complexes with neutral and an...

Nucleus-Independent Chemical Shifts from Semiempirical Calculations

A recently developed special MNDO parameterization for NMR chemical shifts is used to compute the nucleus-independent chemical shifts (NICS) for a wide range of organic molecules, including [n]annulenes, polycyclic hydrocarbons, heterocycles, cage molecules, fullerenes, and pericyclic transition states. The results are compared with published NICS data from ab initio and density functional calculations. In general, there is reasonable agreement. The semiempirical NICS values tend to be smaller in absolute value than their ab initio counterparts, but they often show similar trends. The aromatic or antiaromatic character of a given system can normally be assigned correctly on the basis of the MNDO NICS values.

Competition between predissociative and radiative decays in thee3Σu+andd3Πu−states ofH2andD2

Experimental and theoretical lifetimes ${\ensuremath{\tau}}_{{v}^{\ensuremath{'}}{N}^{\ensuremath{'}}}$ for $e{}^{3}{\ensuremath{\Sigma}}_{u}^{+}$ and $d{}^{3}{\ensuremath{\Pi}}_{u}^{\ensuremath{-}}$ states of molecular hydrogen and deuterium are presented in a wide range of vibrational ${v}^{\ensuremath{'}}$ and rotational ${N}^{\ensuremath{'}}$ quantum numbers. Lifetimes have been measured by a delayed coincidence method following electron-impact excitation from the ground state. The cascade effects on the lifetimes measured were estimated to be negligible through a detailed analysis. Predissociative rates were estimated by the Fermi golden rule based on adiabatic rovibronic wave functions and radial coupling matrix elements $B(R)$ for the $e{}^{3}{\ensuremath{\Sigma}}_{u}^{+}\ensuremath{\sim}b{}^{3}{\ensuremath{\Sigma}}_{u}^{+}$ and $d{}^{3}{\ensuremath{\Pi}}_{u}^{\ensuremath{-}}\ensuremath{\sim}c{}^{3}{\ensuremath{\Pi}}_{u}^{\ensuremath{-}}$ pairs of interacting states. The radiative lifetimes of both excited states were evaluated by the theoretical $e{}^{3}{\ensuremath{\Sigma}}_{u}^{+}\ensuremath{-}a{}^{3}{\ensuremath{\Sigma}}_{g}^{+}$ and $d{}^{3}{\ensuremath{\Pi}}_{u}--a{}^{3}{\ensuremath{\Sigma}}_{g}^{+}$ transition dipole moment functions $d(R)$ and adiabatic rovibronic wave functions of the states involved. The electronic matrix elements $B(R)$ and $d(R)$ were obtained by direct ab initio calculations as well as within the framework of quantum defect theory. The ab initio $B(R)$ functions were calculated by the finite-difference method and by the analytical Sidis formula, which were based on the full configuration-interaction electronic wave functions. At small and medium internuclear distances the quantum defect matrix elements agree well with their ab initio counterparts. The predissociative rates for both $e{}^{3}{\ensuremath{\Sigma}}_{u}^{+}{(v}^{\ensuremath{'}}>~0)$ and $d{}^{3}{\ensuremath{\Pi}}_{u}^{\ensuremath{-}}{(v}^{\ensuremath{'}}>~4)$ states of hydrogen were found to increase sharply as the ${v}^{\ensuremath{'}}$ value of the predissociated level increases, and become compatible with the radiative decay rate. The predissociation effect in the deuterium was negligible for both states studied; hence their experimental lifetimes correspond to the radiative decay channel only. A sum of the theoretical radiative and predissociative rates was in good agreement with the experimental results for the $e{}^{3}{\ensuremath{\Sigma}}_{u}^{+}$ state but slightly overestimated for the $d{}^{3}{\ensuremath{\Pi}}_{u}^{\ensuremath{-}}$ state. Reasons for the remaining discrepancies are discussed and suggestions for further improvements are given.

Effective Hamiltonians of polymethineimine, polyazine and polyazoethene: A density matrix variation approach

A new variation method is proposed to determine the effective Hamiltonians for conjugated π-electron systems. This method is based on the minimization of the difference between the ground state reduced single electron density matrix calculated from the effective Hamiltonian and its ab initio counterpart under a set of well-defined constraints. Applications are made to various oligomers of polymethineimine (PMI), polyazine (PAZ) and polyazoethene (PAE) at the Hartree–Fock level. Calculated are also the optical gaps of these oligomers. The effective Hamiltonians contain electron–electron Coulomb interactions and are suitable for the study of excited state dynamic processes such as nonlinear optical properties in π-conjugated systems.

The effective fragment model for solvation: Internal rotation in formamide

The internal rotation of formamide with 0–5 water molecules oriented along the N–C bond has been studied by the full ab initio self-consistent field theory and using the effective fragment (EFP) method. For each case, the EFP geometries, harmonic vibrational frequencies, rotational barriers, and intrinsic reaction coordinates for the internal rotation are found to be in excellent agreement with their ab initio counterparts. The global energy minimum structures for four and five water complexes are predicted to be formamide bonded to two adjacent waters, with all water molecules in a ring. Probably due to the structural constraints, the complexes containing less than four waters have cyclic structures with the two ends of formamide connected by a sequence of water molecules. The internal rotation barrier of formamide–water complexes increases from 15.3 kcal/mol with no water to 19.0 kcal/mol with four waters and seems to saturate at four to five waters. When electron correlation corrections are added, the estimated internal rotation barrier is ∼20 kcal/mol, in very good agreement with experimental measurements.

Ab initio computation of semiempirical π-electron methods. III. The benzene molecule, the zero-differential-overlap approximation, and the transferability of parameters

The correlated, size-extensive ab initio effective valence shell Hamiltonian (ℋν) method is used to compute the true π-electron Hamiltonian of benzene and to use it for testing various assumptions of the Pariser–Parr–Pople (PPP) semiempirical electronic structure method. On one hand, the ab initio ℋν method enables computing the low lying valencelike vertical excitation energies and ionization potentials. For example, the calculated ℋν excitation energies deviate from experiment on average by only 0.28 eV, which compares well with similar correlated ab initio calculations. More remarkably, however, the ℋν method can reproduce the vertical excitation energies on average to 0.35 eV simply by employing valence orbitals constructed from symmetry adapted linear combinations of carbon atom pπ orbitals. Thus the ℋν calculations demonstrate that accurate benzene excitation energies can be obtained using a common set of valence orbitals for every valencelike state and, in fact, for every conjugated hydrocarbon! In addition, the true, correlated benzene π-electron effective integrals are ab initio counterparts of semiempirical parameters and transfer extremely well to other small molecules such as ethylene and cyclo-butadiene. The degree of transferability is improved when increasing the size of the primitive ab initio basis set. An analysis of the ℋν effective integrals demonstrates that the true PPP model can indeed neglect a portion of one- and two-electron parameters which are assumed to be small by the zero differential overlap (ZDO) approximation, since the correlation contributions render some effective integrals negligible. On the other hand, some typically neglected parameters, such as two-electron, three-center interactions, are computed as too large to just neglect. Likewise, the exact π-electron ℋν contains three-electron many-body interactions which are large (greater than 0.1 eV) and thus cannot be explicitly neglected in PPP models.