Doubles Configuration Interaction Research Articles

Single-photon ionization of SiC in the gas phase: experimental and ab initio characterization of SiC.

We report the first experimental observation of single-photon ionization transitions of the SiC radical between 8.0 and 11.0 eV performed on the DESIRS beamline at the SOLEIL synchrotron facility. The SiC radical, very difficult to synthesize in the gas phase, was produced through chemical reactions between CHx (x = 0-3) and SiHy (y = 0-3) in a continuous microwave discharge flow tube, the CHx and SiHy species being formed by successive hydrogen-atom abstractions induced by fluorine atoms on methane and silane, respectively. Mass-selected ion yield and photoelectron spectra were recorded as a function of photon energy using a double imaging photoelectron/photoion coincidence spectrometer. The photoelectron spectrum enables the first direct experimental determinations of the X+ 4Σ- ← X 3Π and 1+ 2Π ← X 3Π adiabatic ionization energies of SiC (8.978(10) eV and 10.216(24) eV, respectively). Calculated spectra based on Franck-Condon factors are compared with the experimental spectra. These spectra were obtained by solving the rovibrational Hamiltonian, using the potential energy curves calculated at the multireference single and double configuration interaction level with Davidson correction (MRCI + Q) and the aug-cc-pV5Z basis set. MRCI + Q calculations including the core and core-valence electron correlation were performed using the aug-cc-pCV6Z basis set to predict the spectroscopic properties of the six lowest electronic states of SiC+. Complete basis set extrapolations and relativistic energy corrections were also included in the determination of the energy differences characterizing the photoionization process. Using our experimental and theoretical results, we derived semi-experimental values for the five lowest ionization energies of SiC.

Beyond the Ground State: Predicting Electron Ionization Mass Spectra Using Excited-State Molecular Dynamics.

Here, we provide an algorithm that introduces excited states into the molecular dynamics prediction of the 70 eV electron ionization mass spectra. To decide the contributions of different electronic states, the ionization cross section associated with relevant molecular orbitals was calculated by the binary-encounter-Bethe (BEB) model. We used a fast orthogonalization model/single and double state configuration interaction (OM2/CISD) method to implement excited states calculations and combined this with the GFN1-xTB semiempirical model. Demonstrated by predicting the mass spectrum of urocanic acid, we showed better accuracies to experimental spectra using excited-state molecular dynamics than calculations that only used the ground-state occupation. For several histidine pathway intermediates, we found that excited-state corrections yielded an average of 73% more true positive ions compared to the OM2 method when matching to experimental spectra and 16% more true positive ions compared to the GFN method. Importantly, the exited state models also correctly predict several fragmentation reactions that were missing from both ground-state methods. Overall, for 48 calculated molecules, we found the best average mass spectral similarity scores for the mixed excited-state method compared to the ground-state methods using either cosine, weighted dot score, or entropy similarity calculations. Therefore, we recommend adding excited-state calculations for predicting the electron ionization mass spectra of small molecules in metabolomics.

Effect of spin–orbit coupling on strong field ionization simulated with time-dependent configuration interaction

Time-dependent configuration interaction with a complex absorbing potential has been used to simulate strong field ionization by intense laser fields. Because spin-orbit coupling changes the energies of the ground and excited states, it can affect the strong field ionization rate for molecules containing heavy atoms. Configuration interaction with single excitations (CIS) has been employed for strong field ionization of closed shell systems. Single and double excitation configuration interaction with ionization (CISD-IP) has been used to treat ionization of degenerate states of cations on an equal footing. The CISD-IP wavefunction consists of ionizing single (one hole) and double (two hole/one particle) excitations from the neutral atom. Spin-orbit coupling has been implemented using an effective one electron spin-orbit coupling operator. The effective nuclear charge in the spin-orbit coupling operator has been optimized for Ar+, Kr+, Xe+, HX+ (X = Cl, Br, and I). Spin-orbit effects on angular dependence of the strong field ionization have been studied for HX and HX+. The effects of spin-orbit coupling are largest for ionization from the π orbitals of HX+. In a static field, oscillations are seen between the 2Π3/2 and 2Π1/2 states of HX+. For ionization of HX+ by a two cycle circularly polarized pulse, a single peak is seen when the maximum in the carrier envelope is perpendicular to the molecular axis and two peaks are seen when it is parallel to the axis. This is the result of the greater ionization rate for the π orbitals than for the σ orbitals.

Molecular Opacity Calculations for Lithium Hydride at Low TemperatureSupported by the National Key Research and Development Program of China (Grant No. 2017YFA0402300), the National Natural Science Foundation of China (Grant Nos. 11934004 and 11604052), and the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province, China (Grant No. 135409230).

The opacities of the lithium hydride molecule are calculated for temperatures of 300 K, 1000 K, 1500 K, and 2000 K, at a pressure of 10 atm, in which the contributions from the five low-lying electronic states are considered. The ab initio multi-reference single and double excitation configuration interaction (MRDCI) method is applied to compute the potential energy curves (PECs) of the 7LiH, including four 1 Σ + states and one 1 Π state, as well as the corresponding transition dipole moments between these states. The ro-vibrational energy levels are calculated based on the PECs obtained, together with the spectroscopic constants. In addition, the partition functions are also computed, and are provided at temperatures ranging from 10 K to 2000 K for 7LiH, 7LiD, 6LiH, and 6LiD.

Comparative metathermodynamic description of thermal and correlation electron effects

Unified metathermodynamic description of thermal and correlation electron effects is proposed. It is based on the temperature-dependent populations of the common for all temperatures basis states of the (formally accurate) configuration interaction (CI) expansion of the components of the canonical ensemble. These populations are incorporated into the effective ensemble of the metathermodynamic approach through the partial effective tem- peratures. The extended temperature ${T}_{e}$ is introduced as the statistical average of those temperatures. The absolute zero of the ${T}_{e}$ scale represents a ``no-correlation'' Hartree-Fock state, while elevated ${T}_{e}$ describes the increase of the ``correlation motion'' and the conventional thermal motion. The diagonal double configuration interaction (DDCI) approximation is considered, in which the proposed metathermodynamic description of thermal and correlation electron effects is based on the temperature-dependent orbital populations.

Angular dependence of strong field sequential double ionization for neon and acetylene simulated with time-dependent configuration interaction using CIS and CISD-IP.

Strong field ionization is fundamentally important for attosecond spectroscopy and coherence control. However, the modeling beyond the single active electron approximation is still difficult. Time-dependent configuration interaction with singly excited configurations and a complex absorbing potential (TDCIS-CAP), can be used to simulate single and double ionization by intense laser fields. When the monocation does not have degenerate states, TDCIS-CAP starting from a Hartree-Fock calculation of the cation is suitable for simulating the second ionization step. When the monocation has two or more degenerate states, the simulations should treat these degenerate states equivalently. CISD-IP (single and double excitation configuration interaction with ionization) can be used to treat degenerate states of the cation on an equal footing by representing the cation wavefunctions with ionizing single (1 hole) and double (2 holes/1 particle) excitations from the neutral molecule. Since CISD-IP includes single excitations for each of the monocation states, time dependent CISD-IP with a complex absorbing potential (TDCISDIP-CAP) can also be used to simulate ionization to the dications states. In this work, TDCIS-CAP and TDCISDIP-CAP have been used to simulate the angular dependence of ionization of the neon cation and acetylene cation. In both cases, the second electron is ionized predominantly from an orbital perpendicular to the orbital involved in the first ionization. The TDCISDIP-CAP simulations show some features involving interactions between the monocation states that are not seen in the TDCIS-CAP simulations.

Jahn-Teller and Pseudo Jahn-Teller Effects: Influences on the Electronic Structures of Small Transition, Main Group and Mixed Metal Clusters

The present review article focuses on the impact of the Jahn-Teller effect (JTE) including hidden JTE and pseudo JTE on the ground and low-lying electronic structures of small transition, main group, and mixed metal clusters. The discussions are based on the results acquired from the quantum chemical analysis involving complete active space multiconfiguration self-consistent field (CASMCSCF or CASSCF) together with multireference singles and doubles configuration interaction (MRSDCI) theories. Related computational results from density functional (DFT) and coupled cluster (CC) theories, and the available experimental evidences on the JT-distorted structures are also included for comparative analysis. The role of the relativistic effects on several heavy metal clusters is discussed in detail, since these effects could lead to either enhancement or the quenching of the JTE. The final section of the review manifests the JT-distortion of several mixed metal clusters of icosahedral symmetry and they are explained through a much simpler shell structure (jellium) model.

Theoretical electronic structure with rovibrational calculations of the alkali chloride molecules XCl (X = Li, Na, K, Rb, Cs)

The adiabatic potential energy curves and the static dipole moment curves of the low-lying singlet and triplet electronic states in the representation 2s+1Λ(+/−) of the five alkali chloride molecules (LiCl, NaCl, KCl, RbCl and CsCl) have been investigated via ab initio calculations using the state averaged complete active space self consistent field followed by the Multi-reference single and double configuration interaction method with Davidson correction CASSCF/(MRCI + Q). For the molecules under consideration, the spectroscopic constants Te, Re, ωe and Be, the dipole moment μe, and the dissociation energy De have been calculated for the bound states along with the percentage ionic character fionic around the equilibrium position of the ground state. Moreover, the transition dipole moment curves of the X1∑+–(2)1∑+ and X1∑+–(1)1Π transitions have been investigated for the five alkali chloride molecules. A rovibrational calculation was carried out using the canonical functions approach in order to determine the rovibrational constants Ev, Bv and Dv and the abscissas of the turning points Rmin and Rmax for the investigated bound states.

A matrix isolation ESR investigation of Mg+-N2.

The adducts formed between 25Mg+ with 14N2 and 25Mg+ with 15N2 have been trapped in a solid neon matrix and studied with electron spin resonance (ESR) spectroscopy. These radical species were formed through the interaction of laser ablated magnesium and nitrogen gas. The Mg+-N2 radical species was found to have a ground electronic state of 2Σ+ in a linear configuration with discrete coupling to the proximate nitrogen resolved in the spectra. Fitting the ESR spectra allowed magnetic parameters to be determined as follows: g⊥ = 2.0012(5), g∥ = 2.0015(8), A⊥(1-14N) = 32(3) MHz, A∥(1-14N) = 34(5) MHz, A⊥(1-15N) = 45(4) MHz, A∥(1-15N) = 47(6) MHz, A⊥(25Mg) = -581(5) MHz, and A∥(25Mg) = -582(5) MHz, and estimates derived for A⊥(2-14N) = 1(2) MHz, A∥(2-14N) = 2(5) MHz, A⊥(2-15N) = 2(2) MHz, and A∥(2-15N) = 4(6) MHz. Ab initio calculations using the coupled-cluster single double triple methodology showed that the linear form was 59.7 kcal mol-1 more stable than the T-shaped form. The potential energy curve around the equilibrium geometry was explored using the complete active space self-consistent field approach, and Hartree-Fock singles and double configuration interaction and multireference singles and double configuration interaction calculations of the hyperfine coupling constants were undertaken, and reasonable agreement with the experiment was observed.

Neural network diabatization: A new ansatz for accurate high-dimensional coupled potential energy surfaces.

A new diabatization method based on artificial neural networks (ANNs) is presented, which is capable of reproducing high-quality ab initio data with excellent accuracy for use in quantum dynamics studies. The diabatic potential matrix is expanded in terms of a set of basic coupling matrices and the expansion coefficients are made geometry-dependent by the output neurons of the ANN. The ANN is trained with respect to ab initio data using a modified Marquardt-Levenberg back-propagation algorithm. Due to its setup, this approach combines the stability and straightforwardness of a standard low-order vibronic coupling model with the accuracy by the ANN, making it particularly advantageous for problems with a complicated electronic structure. This approach combines the stability and straightforwardness of a standard low-order vibronic coupling model with the accuracy by the ANN, making it particularly advantageous for problems with a complicated electronic structure. This novel ANN diabatization approach has been applied to the low-lying electronic states of NO3 as a prototypical and notoriously difficult Jahn-Teller system in which the accurate description of the very strong non-adiabatic coupling is of paramount importance. Thorough tests show that an ANN with a single hidden layer is sufficient to achieve excellent results and the use of a "deeper" layering shows no clear benefit. The newly developed diabatic ANN potential energy surface (PES) model accurately reproduces a set of more than 90 000 Multi-configuration Reference Singles and Doubles Configuration Interaction (MR-SDCI) energies for the five lowest PES sheets.

Diffusion and reptation quantum Monte Carlo study of the NaK molecule

ABSTRACTDiffusion Monte Carlo (DMC) and Reptation Monte Carlo (RMC) methods, have been applied to study some properties of the NaK molecule. Hartree–Fock (HF), Density Functional (DFT) and single and double configuration interaction (SDCI) wavefunctions with a valence quadruple zeta atomic natural orbital (VQZ/ANO) basis set were used as trial wavefunctions. Values for the potential energy curve, dissociation energy and dipole moment were calculated for all methods and compared with experimental results and previous theoretical derivations. Quantum Monte Carlo (QMC) calculations were shown to be useful methods to recover correlation in NaK, essential to obtain a reasonable description of the molecule. The equilibrium distance yield a value close to the experimental one. The dissociation energy, however, is not so good, and the minimum error is about 4% of the experimental value, except when a coupled-clusters method is used. In this case, a conventional CCSD(T) calculation with an extended aug-pc-4 basis set gives a much better agreement to experiment. However, neither the CCSD(T) nor other MO and DFT methods are able to reproduce correctly the large dipole moment of this molecule. Even DMC methods with a simple HF trial wavefunction are able to give a better agreement to experiment, although they have a larger error bar.

Determining whether diabolical singularities limit the accuracy of molecular property based diabatic representations: The 1,21A states of methylamine.

An efficient, easily implemented method for locating singularities attributable to the failure of the defining equations in a molecular property based diabatization, termed diabolical singular points, is reported. For two state diabatizations, the singular points form a seam of dimension N int - 2, where N int is the number of internal degrees of freedom. The dynamical outcomes of nuclear trajectories that reach the region of this seam are flawed. The algorithm easily identifies these otherwise hard to anticipate regions of fallaciously large derivative coupling. The fact that the algorithm is easily incorporated into a two state diabatization code based on molecular properties makes it a practical tool for determining whether the existence of diabolical singularities is relevant to the problem being considered. The algorithm is illustrated using a multireference single and double excitation configuration interaction description of the 1,21A states of CH3NH2.

$$\hbox {H}^{+} + \hbox {O}_2$$ H + + O 2 system revisited: four-state quasidiabatic potential energy surfaces and coupling potentials

The global adiabatic and quasidiabatic potential energy surfaces for the ground and first three excited ( $$1-4\, ^3\text {A}^{\prime \prime }$$ ) electronic states of $$\hbox {H}^{+} +\hbox {O}_2$$ system are reported on a finer grid points in the Jacobi coordinates using Dunning’s cc-pVTZ basis set and internally contracted multi-reference (single and double) configuration interaction method. Ab initio procedures have been used to compute the corresponding quasidiabatic surfaces and radial coupling potentials which are relevant for the dynamical studies of inelastic vibrational excitation and charge transfer processes. Nonadiabatic couplings arising out of relative motion of proton and the vibrational motions of $$\hbox {O}_2$$ between the adiabatic electronic states have also been analyzed. Adiabatic and quasidiabatic potential energy surfaces have been constructed using ab initio procedure for the lowest four 1-4 $$^{3}A''$$ electronic states of $$\hbox {H}^{+}$$ + $$\hbox {O}_{2}$$ collision system. The computed quasidiabatic surfaces and coupling potentials would be helpful in understanding the collision dynamics of inelastic and charge transfer processes in the system.

FIRST-PRINCIPLES SIMULATION OF THE CERIUM TRIFLUORIDE INFRARED SPECTRUM BEYOND THE BORN–OPPENHEIMER APPROXIMATION

The group-theoretical analysis was carried out to derive a spin-vibronic model Hamiltonian and a diabatic dipole moment operator for a cerium trihalide molecule, CeX3. The model comprises seven lowest-lying 4f1 electronic states coupled by six vibrational modes, (A2'' + E' + E'' + A1' + A2') × (a1' + a2'' + e' + e'), with an accounting for spin-orbit coupling. Multimode potential energy surfaces have been calculated for the CeF3 molecule at the multi-reference singles and doubles configuration interaction level of theory corrected for quadruple excitations, MRCISD+Q, and dipole moment surfaces at the MRCISD level. A hybrid approach employing a quasi-diabatization technique was utilized to determine the relevant model parameters up to forth order in power series of the Q1(a1'), Q3(e'), Q4(e') normal coordinates, and eighth order of the Q2(a2'') out-of-plane bending normal coordinate. Spin-orbit coupling was taken into account through zero-order, with the respective constants obtained from matrix elements of the Breit–Pauli operator in the basis of states generated at the MRCISD level. The spin-vibronic Hamiltonian and dipole moment operator obtained in this way has been utilized in the variational calculations to simulate the infrared absorption spectrum of CeF3. The resulting spectrum features a complex structure owing to an intricate interplay of the vibronic (Jahn–Teller and pseudo-Jahn–Teller) and spin-orbit coupling effects, and hence cannot be explained within the conventional Born–Oppenheimer (BO) approximation. The strongest absorption appearing in the simulated spectrum at about 500 cm–1, mostly associated with the Q3(e') stretching mode, is split into two bands by about 3 cm–1. This finding is in full agreement with the CeF3 matrix isolation infrared spectroscopy data. On the whole, the results of the calculations clearly indicate the vibronic rather than vibrational origin of the spectral bands, including the bands in the low-frequency region of the spectrum, and thus show the fallacy of the generally accepted assignment of the bands observed in the experiment to the fundamental vibrational transitions of the molecule made on the assumption of the admissibility of describing this within the Born–Oppenheimer approximation. The new assignment of the experimental IR spectrum of CeF3 is proposed.Forcitation:Vasilyev O.A., Solomonik V.G. First-principles simulation of the cerium trifluoride infrared spectrum beyond the Born–Oppenheimer approximation. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2018. V. 61. N 3. P. 31-44

Ab initio potential energy surfaces of HCS+: A study of the ground and the low-lying excited electronic states

Three dimensional ab initio potential energy surfaces (PESs) have been computed for the ground state and low-lying excited states of HCS+ molecular ion using the internally contracted multi-reference (single and double) configuration interaction and augmented correlation consistent polarized valence quadruple zeta (aug-cc-pVQZ) basis sets. Ground state global PES is analyzed as dissociation of molecular ion into H+CS+. The ground state PES (H+CS+) has been fitted by the inverse power series expansion function. The anisotropy of the surface has been analyzed in terms of the multipolar expansion coefficients for the rigid-rotor surface. The surface will be useful for detailed understanding of collision dynamics in terms of ro-vibrational cross sections and rate coefficients.

BASF accelerating and focusing research activities

Spectroscopic constants and potential energy curves of several electronic states of PCl2 (X̃ 2B1, 2A1(I), 2A1(II), 2A2, 2Pg, 4Pg, 2B24Σg−, 4B1, 2B1(II)), 2B1(III)) and PCl2+ (1A1, 3B1 and 1B1) have been investigated using the complete active space self-consistent field (CASSCF) followed by multi-reference singles and doubles configuration interaction (MRSDCI) methods that included up to 1.2 million configurations. The bond dissociation energies of PCl and PCl2 as well as the adiabatic ionization energy of PCl2 are computed. The observed A−X system is reassigned to the Ã2B2−X̃ 2B1 transition.

Molecular electric moments calculated by using natural orbital functional theory.

The molecular electric dipole, quadrupole, and octupole moments of a selected set of 21 spin-compensated molecules are determined employing the extended version of the Piris natural orbital functional 6 (PNOF6), using the triple-ζ Gaussian basis set with polarization functions developed by Sadlej, at the experimental geometries. The performance of the PNOF6 is established by carrying out a statistical analysis of the mean absolute errors with respect to the experiment. The calculated PNOF6 electric moments agree satisfactorily with the corresponding experimental data and are in good agreement with the values obtained by accurate ab initio methods, namely, the coupled-cluster single and doubles and multi-reference single and double excitation configuration interaction methods.

Combined theoretical and experimental study of the valence, Rydberg and ionic states of fluorobenzene.

New photoelectron spectra (PES) and ultra violet (UV) and vacuum UV (VUV) absorption spectra of fluorobenzene recorded at higher resolution than previously, have been combined with mass-resolved (2 + 1) and (3 + 1) resonance enhanced multiphoton ionization (REMPI) spectra; this has led to the identification of numerous Rydberg states. The PES have been compared with earlier mass-analyzed threshold ionization and photoinduced Rydberg ionization (PIRI) spectra to give an overall picture of the ionic state sequence. The analysis of these spectra using both equations of motion with coupled cluster singles and doubles (EOM-CCSD) configuration interaction and time dependent density functional theory (TDDFT) calculations have been combined with vibrational analysis of both the hot and cold bands of the spectra, in considerable detail. The results extend several earlier studies on the vibronic coupling leading to conical intersections between the X(2)B1 and A(2)A2 states, and a further trio (B, C, and D) of states. The conical intersection of the X and A states has been explicitly identified, and its structure and energetics evaluated. The energy sequence of the last group is only acceptable to the present study if given as B(2)B2<C(2)B1<D(2)A1, a conclusion which is in agreement with most previous EOM-CCSD and other calculations. However, this symmetry ordering of the B and C states forces reconsideration of the nature of the PIRI spectrum. The coupling between these two states is induced by the a2 modes, ν12 and ν14 and we propose that the 14(1) band is observed in the B(2)B2 band in the PES for the first time, because of the improved resolution. This same assignment is given to the lowest energy band in the PIRI spectrum which was previously assigned as the origin band and further conclude that the entire PIRI spectrum is induced by ν12 and ν14. The relative intensities of the various Rydberg state peaks in the VUV absorption and REMPI spectra of fluorobenzene are very similar to those observed in the equivalent spectra of benzene.

Rydberg, valence, and ion-pair quintet states of O2.

We report an ab initio study of the quintet states of molecular oxygen. The calculations are carried out employing the multireference single and double excitation configuration interaction package. Potential energy curves of the six quintet valence states dissociating into ground state atoms and of the four quintet states dissociating to ion-pair atoms were computed. A number of bound quintet Rydberg series converging to the a(4)Πu and b(4)Σg(-) states of the O2(+) cation have been identified.

An improved quasi-diabatic representation of the 1, 2, 3(1)A coupled adiabatic potential energy surfaces of phenol in the full 33 internal coordinates.

In a recent work we constructed a quasi-diabatic representation, H(d), of the 1, 2, 3(1)A adiabatic states of phenol from high level multireference single and double excitation configuration interaction electronic structure data, energies, energy gradients, and derivative couplings. That H(d) accurately describes surface minima, saddle points, and also regions of strong nonadiabatic interactions, reproducing the locus of conical intersection seams and the coordinate dependence of the derivative couplings. The present work determines the accuracy of H(d) for describing phenol photodissociation. Additionally, we demonstrate that a modest energetic shift of two diabats yields a quantifiably more accurate H(d) compared with experimental energetics. The analysis shows that in favorable circumstances it is possible to use single point energies obtained from the most reliable electronic structure methods available, including methods for which the energy gradients and derivative couplings are not available, to improve the quality of a global representation of several coupled potential energy surfaces. Our data suggest an alternative interpretation of kinetic energy release measurements near λphot ∼ 248 nm.