Multiconfigurational Quasidegenerate Perturbation Theory Research Articles

Multiconfigurational Self-Consistent Field Study of the Excited State Properties of 4-Thiouracil in the Gas Phase

A comprehensive theoretical study of the vertical singlet and triplet electronic transitions of 4-thiouracil was performed at the multiconfigurational self-consistent field (MCSCF) level of theory. The ground state geometry of the molecule was optimized at the MP2/6-311++G(d,p) level. The MCSCF calculations were performed using the 6-311+G(d) basis set. The active space for the MCSCF calculations consisted of 12 orbitals in which 6 orbitals were the occupied π type while the remaining 6 orbitals were the virtual π* type. To compute the nπ* transitions, the two occupied orbitals were replaced with two σ orbitals localized at the thiocarbonyl and carbonyl groups, respectively. Further, MCSCF calculations were also performed with a slightly smaller active space where 10 electrons were distributed in 11 orbitals. The effects of dynamic electron correlation on the MCSCF energies were considered at the second-order multiconfigurational quasi-degenerate perturbation (MCQDPT2) theory. The computed transition energies after the MCQDPT2 correlation were found to be in agreement with the experimental data. The ground and excited state molecular electrostatic potentials and the Mulliken charge distributions in different states were also investigated.

Nonempirical Description of the Atmospherically Important Anionic Species. II. Hydrated Ozone Anions

Ozone–water clusters are nonempirically modeled in the complete active space self-consistent field approximation (CASSCF) with the energetic estimates obtained at the multiconfiguration quasidegenerate perturbation theory level (MCQDPT) with 6−31++G** basis set. Coordination of a neutral ozone molecule to small water clusters is either surface or interior, with the binding energy of the order of a weak hydrogen bond. Upon localization of an excess electron, the hydration of ozone becomes strong. The adiabatic affinities of water–ozone clusters and the energies of electron detachment from their anions, depending on the number of water molecules, estimate the electron hydration and vertical electron detachment thresholds of water or ice that superficially coordinates minor amounts of ozone.

Photoexcited O3(H2O)n cluster decay: Nonempirical simulation of model systems

AbstractThe ground and excited electronic states of an individual ozone molecule and its clusters with up to five water molecules are considered at the multiconfigurational quasidegenerate perturbation theory level with the geometrical optimization of structures carried out in the multiconfigurational self‐consistent field approximation in complete active space with 6‐31G basis set extended with diffuse and polarization functions on all nuclei. The vertical photoexcitation spectra of an ozone molecule and ozone–water clusters in the energy range up to 6.0 eV are compared. An interaction between singlet dissociation products of ozone and either a single water molecule or a small water cluster is considered, and the formation of radical species is predicted. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

A multi-configurational study of the one-dimensional dissociation of azidopentazole (N 8) and derived N 7CH isomers

One-dimensional dissociation curves in N 8 and derived N 7CH were calculated at the ab initio complete active space self-consistent field and multi-configuration quasidegenerate perturbation theory levels using the 6-311G(d) basis set. A limited active space of 12 electrons distributed among 11 molcular orbitals was used. The calculated barrier heights, either intrinsic or reduced by a triplet state crossing, are deemed to be too small to support either N 8 or N 7H as a high energy density material. The limited size of the active space and basis set used here caused discontinuities and different asymptotic fragmentations in some of the dissociation curves.

Theoretical Study of the pH-Dependent Photophysics Of N1,N6-Ethenoadenine and N3,N4-Ethenocytosine

N1,N6-ethenoadenine (N1,N6-εA) and N3,N4-ethenocytosine (N3,N4-εC) nucleos(t)ides are widely used fluorescent biochemical probes. Interestingly, the fluorescence of these compounds is highly dependent on the acidity of the medium. It is well established that the neutral form of the N1,N6-εA chromophore is responsible for its fluorescence, whereas the protonated form of N3,N4-εC is slightly fluorescent. However, the origin of the pH-dependent fluorescence has remained elusive. In the current work, the electronic excitation and emission gas-phase spectra of 3-Me-N1,N6-εA (1) and 1-Me-N3,N4-εC (2) were calculated using configuration interaction singles (CIS), time dependent density functional theory, and multiconfigurational quasidegenerate second-order perturbation theory methods. The solvatochromic shift of 1 and 2 due to hydration was computed using a stepwise molecular dynamics-semiempirical CIS method. Good agreement between the computed and experimental spectra was obtained, both in the gas phase and in aqueous solution. Our results suggest that the pH-dependent loss of fluorescence of 1 is due to interaction between the lowest ππ* and nπ* excited states, which may lead to rapid radiationless decay to the ground state.

Electronic Spectra, Excited-State Geometries, and Molecular Electrostatic Potentials of Hypoxanthine: A Theoretical Investigation

The ground-state geometry of hypoxanthine was optimized at the MP2, B3LYP, and HF levels by employing the 6-311++G(d,p) basis set. The vertical singlet transition energies were calculated at the CASSCF/ 6-31+G(d), TD-B3LYP/6-311++G(d,p), and CIS/6-311++G(d,p) levels by using the MP2-, B3LYP-, and Hartree−Fock-optimized geometries, respectively. In the case of the CASSCF calculations, the active space consisted of the 2σ, 6π, and 4π* orbitals. The σ orbitals were used to compute the nπ* transitions. The effects of dynamic correlation on the CASSCF energies were considered at the second-order multiconfigurational quasi-degenerate perturbation (MCQDPT2) theory. The effect of hydration was considered by including three water molecules in the first solvation shell of hypoxanthine. The geometry of the molecule was also optimized in the lowest singlet ππ* and nπ* excited states at the CIS/6-311++G(d,p) level. The characteristics of the ground and excited-state potential energy surfaces were ascertained from a ha...

Ab initio calculations on the Ground and Low‐Lying Excited States of InH.

Using multiconfiguration quasi-degenerate perturbation theory (MCQDPT), the ground and the low-lying excited states of InH are studied, and the potential energy curves and the spectroscopic constants of the , a 3Π0−,0+,1,2 and A 1Π1 states are obtained. By comparing the result of all-electron basis sets with that of relativistic effective core potential (RECP) basis sets, it is found that the former is better than the latter, especially in describing the A 1Π1 state. The transition properties of the excited states, including the transition dipole moments, the radiative lifetimes and the Franck–Condon factors, are predicted. It is shown that a 3Π0+ and a 3Π1 are long-lived states, while A 1Π1 is a short-lived state.

An ab initio study of excited states of U and UF

By using the third-order Douglas-Kroll method and spin–orbit multiconfiguration quasidegenerate perturbation theory we have been able to predict the spectrum of odd parity states of uranium atom within at worst 1000–2000 cm−1 accuracy for the 48 states presented. For UF we have given Morse potential parameters for the lowest 18 Ω states. The ground state was found to be I9/24. The importance of dynamic correlation effects including core correlation treated on the same footing with spin–orbit coupling is elucidated.

Ab initio calculations on the ground and low-lying excited states of InH

Using multiconfiguration quasi-degenerate perturbation theory (MCQDPT), the ground and the low-lying excited states of InH are studied, and the potential energy curves and the spectroscopic constants of the , a 3Π0−,0+,1,2 and A 1Π1 states are obtained. By comparing the result of all-electron basis sets with that of relativistic effective core potential (RECP) basis sets, it is found that the former is better than the latter, especially in describing the A 1Π1 state. The transition properties of the excited states, including the transition dipole moments, the radiative lifetimes and the Franck–Condon factors, are predicted. It is shown that a 3Π0+ and a 3Π1 are long-lived states, while A 1Π1 is a short-lived state.

Hyperfine coupling tensors for multi-configurational quasi-degenerate perturbation theory

Analytic calculation of the hyperfine coupling tensors for multi-configurational quasi-degenerate perturbation theory is developed based on the Lagrange multiplier formalism. Calculation of Lagrange multipliers is not required if the corresponding constraining conditions do not depend explicitly on the magnetic moments of the nuclei. Except for the explicit form of the one-electron perturbation operator, the derivation presented in this work is also applicable to other molecular properties for which the basis functions do not depend on the differentiation variable, and for which the dependence of the Hamiltonian is through a one-electron operator only.

Ab Initio CASSCF and DFT Investigations of (H2O)2+and (H2S)2+: Hemi-Bonded vs Proton-Transferred Structure

High level ab initio calculations using a complete active space self-consistent field (CASSCF) and multiconfigurational quasi-degenerate perturbation theory (MCQDPT2) methods as well as density functional theory (DFT)-based calculations with different exchange−correlation energy density functionals have been performed for predicting the relative stability of the proton-transferred vs hemi-bonded isomers of (H2O)2+ and (H2S)2+ species. For (H2O)2+, DFT calculation using conventional exchange−correlation functionals predicts the hemi-bonded structure to be the ground state while use of full or half Hartree−Fock exchange and local correlation predicts a higher stability of the proton-transferred structure in agreement with ab initio results. For the (H2S)2+ system, all of the methods lead to the prediction of lower energy for the hemi-bonded isomer. No regular trend of the exchange−correlation energy component with the total energy difference is however observed. Dynamical electron correlation effect incorporated through MCQDPT2 is found to be much stronger in (H2O)2+ as compared to (H2S)2+. An analysis of the nature of interactions involved in the (H2O)2+ and (H2S)2+ systems within the framework of Bader's topological theory of atoms in molecules is also presented through the plots of the Laplacian ∇2ρ of the electron density ρ(r) and also other related quantities at the bond critical points with the objective of rationalizing the relative stability of the two isomers in both (H2O)2+ and (H2S)2+.

Photodissociation of alkyl and aryl iodides and effect of fluorination: Analysis of proposed mechanisms and vertical excitations by spin–orbit ab initio study

An ab initio study of the vertical electronic excitations in CX3I, C6X5H, and C6X5I (X=H and F) is presented. All-electron basis sets are used and the relativistic effects are accounted for with the relativistic elimination of small components scheme. The structures are optimized with the complete active space self-consistent field approach and the excitation energies are computed with the spin–orbit multiconfiguration quasidegenerate perturbation theory. The n–σ* transitions of CX3I, low-lying π–π* transitions of C6X5H, and low-lying n–σ*, π–π*, and π–σ* transitions of C6X5I are elucidated. For CH3I, energy values of parallel and perpendicular transitions differ from experimental values by 455 and 1156 cm−1, respectively. Effects of fluorination are emphasized, it is found that fluorination increases the gap between Q30 and Q11 transitions and increase is substantially more in aryl iodides than in alkyl iodides. Electronic factors influencing increased I* quantum yield in the photodissociation on fluorination of alkyl iodides is attributed to increased gap between Q30 and Q11 transitions reducing curve crossing probability and for aryl iodides there is additional role by phenyl transitions. A correlation diagram illustrating transitions of aryl iodides is presented.

Direct diabatization of electronic states by the fourfold way. II. Dynamical correlation and rearrangement processes

Diabatic representation of coupled potential energy surfaces and their scalar couplings provides a compact and convenient starting point for dynamics calculations carried out in either the adiabatic or diabatic representation. In a previous paper we presented a general, path-independent scheme, called the fourfold way, for calculating diabatic surfaces and their scalar couplings from adiabatic surfaces and electronic density matrices such that the manifold of diabatic states spans the variationally optimized space of a finite number of adiabatic states. In the present paper we extend that scheme in these ways: (1) We show how to include dynamical electronic correlation energy by multireference perturbation theory or configuration interaction based on a complete active reference space. (2) We present a more general strategy for treating rearrangements. (3) We present consistency criteria for testing the validity of the assumptions for a particular choice of reference geometries, diabatic molecular orbital (DMO) ordering, dominant configuration-state-function lists, and choice(s) for reference DMO(s) for systems involving rearrangements. The first extension is illustrated by multiconfiguration quasidegenerate perturbation theory (MC-QDPT) calculations on LiF, and all three extensions are illustrated by MC-QDPT calculations on the reaction Li(2 2S,2 2P)+HF→LiF+H.

Hardness and Polarizability Profiles for Intramolecular Proton Transfer in Water Dimer Radical Cation

In view of the recently reported discrepancies in the prediction of minimum energy structure (proton-transferred vs hemibonded) for the water dimer on ionization, we have performed complete active-space self-consistent field calculations followed by total energy evaluation using multiconfigurational quasi-degenerate perturbation theory to obtain very accurate relative energies of different structures and predicted the proton-transferred structure to be the most stable ones. The variations of hardness, polarizability, chemical potential, and energy for the proton-transfer process in this weakly interacting system (ionized water dimer species) are investigated through calculations using Hartree−Fock theory. It is observed that the transition state corresponding to the proton-transfer process is associated with maximum polarizability at different O−O distances for the water dimer cation, although the hardness minimum does not exactly correspond to the transition state. However, the hardness profiles scaled suitably with chemical potential are found to have minima at the transition states.

Dipole moments of [formula omitted] and [formula omitted] states of CN radical at different internuclear distances via Fock space multi-reference coupled cluster linear response approach

Dipole moments of 2Σ and 2Π states of CN radical at different internuclear distances are reported using the multi-reference coupled cluster (MRCC) linear response approach. We also report as a by product the potential energy surface, equilibrium geometry and harmonic frequency of CN radical states that agree well with those calculated using multi-configuration quasi-degenerate perturbation theory (MCQDPT). The dipole moment of the ground state of the radical is in excellent agreement with the experimental value. Dipole moments are also calculated using MRCI using bigger complete active space and state specific orbitals and results of both methods agree well.

CASSCF Investigation of Electronic Excited States of 2-Aminopurine

2-Aminopurine is a highly fluorescent analogue of adenine that can be incorporated synthetically into DNA with little perturbation of the native double-helical structure. The sensitive dependence of the quantum yield of this fluorophore on nucleic acid conformation has made it an invaluable probe of DNA structure, dynamics, and interactions. To assist in the development of models for the molecular interpretation of fluorescence measurements, the electronic structure of 2-amino-9-methylpurine has been calculated in the ground state and the lowest singlet ππ* and nπ* excited states. These computations employed the complete active space multiconfigurational self-consistent field method (CASSCF), supplemented by multiconfigurational quasi-degenerate perturbation theory (MCQDPT). The predicted energies for ππ* excitation and emission and nπ* excitation are in good agreement with previous experimental values. The permanent molecular dipoles of the ground and ππ* excited states are similar in magnitude and direction, consistent with experimental observations of weak solvatochromic shifts in ππ* absorption and emission spectra. However, the permanent dipole of the nπ* state is rotated approximately 60° relative to that of the ground state, implying that the nπ* excitation energy will increase in more polar solvents due to the relative destabilization of this state by unfavorably oriented solvent dipoles. This result demonstrates that the “blue-shift” of the nπ* state in polar solvents, which is commonly attributed to the effect of hydrogen bonding, can arise entirely from a general solvent effect. The energy of a radiationless vibronic transition from the ππ* state to the nπ* state will increase in more polar solvents, provided that the solvent does not rearrange during the transition. Consequently, the efficiency of fluorescence quenching by vibronic coupling between the ππ* and nπ* states is predicted to decrease significantly in such solvents. The geometry of the fluorescent emitting state, obtained by CASSCF optimization of the ππ* state, is moderately buckled due to the occupation of an antibonding orbital localized to C6. This buckling implies an out-of-plane vibration during the relaxation of the ππ* state, which is required for vibronic coupling between this state and the nπ* state. Such a solvent-sensitive intramolecular quenching mechanism may account for the observed dependence of the fluorescence lifetime of 2-aminopurine on the local environment both in pure solvents and in DNA.

Stability of Hyperlithiated Borides

Structures and energetics of BLin (n = 4−8) clusters are predicted using the SCF, MP2, and B3LYP methods with the 6-31G(d) basis set, including energy evaluations at G2MP2. Cohesive energies, defined as the enthalpies of the BLin → B + Lin reactions, and Li and Li2 elimination reaction enthalpies are also estimated at B3LYP. This level of theory predicts the boron cohesive energy to increase up to the BLi6 cluster after which it levels off. All BLin systems are thermodynamically stable with respect to Li and Li2 dissociations; BLi4 has the largest reaction enthalpies. The energetics of the hyperlithiated borides obtained with B3LYP/6-31G(d) are in reasonable agreement with those at G2MP2 but less satisfactory than those of the smaller BLin (n =1−3) systems. Computations on BLi4 with multiconfigurational quasidegenerate perturbation theory indicate that the B3LYP/6-31G(d) energies may be more reliable for the larger BLin systems.

Ab Initio Design of Two-Photon Absorbing Materials

AbstractTwo-photon absorbing materials such as conjugated polyenes show promise as nonlinear optical materials. Prediction of two-photon absorption frequencies and cross-sections has been limited by the high level of ab initio calculations that must be carried out in order to accurately calculate excited state energies and transition dipole moments, by the size of many of the compounds of interest, and by the difficulty of handling condensed phase effects in the calculations. We have carried out geometry optimizations at the multi-configurational selfconsistent field level on a small polyene, hexatriene, both in the gas-phase and in solution, with the solvent effects being modeled by the effective fragment potential. The excited-state energies have been calculated by the multiconfigurational quasidegenerate perturbation theory. Transition dipole moment calculations have also been carried out, from which the two-photon absorption cross-section can be estimated. The results indicate that just one or two solvent molecules can have a large effect on the nonlinear optical properties of two-photon absorbing materials.

A study of FeCO+ with correlated wavefunctions

A study of FeCO+ and Fe+ using both the second-order multi-configurational quasi-degenerate perturbation theory (MC-QDPT2) method and the coupled cluster theory with single and double replacements (augmented by perturbative triples) [CCSD(T)] method are presented. An all-electron triple-ζ valence plus polarization basis set was used in all calculations. The equilibrium CCSD(T) geometry of FeCO+ is found to be linear (4Σ-) with a Fe+ to CO distance of 1.905 A and a CO bond distance of 1.133 A. The dissociation energy D0 of 4Σ- FeCO+ to 6D Fe+ and 1Σ+ CO is predicted to be 28.8 kcal mol-1, which is within the experimental range. Excited state properties including potential energy surfaces and De are predicted for the low lying sextet and quartet states of FeCO+. The first excited state is predicted to be 4Δ with a De of 17.6 kcal mol-1. The lowest sextet state is predicted to be 6Δ with a D0 of 12.3 kcal mol-1. Several examples of pathological behavior at many levels of theory have been discovered and are discussed.

Reversible Photoinitiated Isomerization Reaction of BrOBr to BrBrO: A Combined Matrix Isolation and ab Initio Study

The reversible photoinitiated isomerization reaction of BrOBr to BrBrO was measured in solid argon matrixes and calculated by multiconfigurational quasidegenerate perturbation theory (MCQDPT). The calculated vertical excitation energies are in good agreement with the experimentally observed excitation wavelengths. While the isomerization from BrOBr to BrBrO appears to be quantitative, some product loss is observed during the back reaction. The calculated excitation energies and oscillator strengths for transitions from the ground to excited states offer a detailed explanation of this behavior.