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Sequential formation of a poly-cyclic aromatic hydrocarbon (PAH) dication in the H I regions of the interstellar medium (ISM) is proposed to be a function of internal energy of the doubly ionized PAHs, which, in turn, is dependent on the single- and double-ionization potentials of the system. This sets a limit on the single- and double-ionization energies of the system(s) that can further undergo sequential absorption of two photons, leading to a dication (PAH+2). Here, we report the single-ionization (I+1) and double-ionization (I+2) energies and the I+2/I+1 ratio for some selected PAHs and conjugated polyenes obtained using the Fock space coupled cluster technique, enabling simultaneous consideration of several electronic states of different characters. The I+2 to I+1 ratio bears a constant ratio, giving allowance to determine I+2 from the knowledge of single-ionization (I+1) and vice versa. Our observations are in good agreement with the established literature findings, confirming the reliability of our estimates. The measured single- and double-ionization energies further demonstrate that the sequential formation and fragmentation of a PAH dication in the H I regions of the ISM for systems such as benzene and conjugated polyenes such as ethylene and butadiene are quite unlikely because I+2–I+1 for such system(s) is higher than the available photon energy in the H I regions of the ISM. Present findings may be useful to understand the formation and underlying decay mechanisms of multiply charged ions from PAHs and related compounds that may accentuate the exploration of the phenomenon of high-temperature superconductivity. [ABSTRACT FROM AUTHOR]

To compute the electronic excitation energies, a state-specific multireference Møller–Plesset perturbation theory (SSMRPT) with a complete active space configuration interaction reference function constructed using the orbitals obtained by the density functional theory (DFT) is presented as an accurate, as well as computationally affordable, and efficient protocol at the level of second order. The global hybrid B3LYP (Becke, 3-parameter, Lee–Yang–Parr) functional has been used to generate orbitals. The present method, called DFT-SSMRPT, uses perturbers that are individual Slater determinants and accounts for the coupling between the nondynamical and dynamical correlation effects. We have applied the new method to compute excitation energies in conjugated systems of π-electrons such as trans-1,3-butadiene, trans,trans-1,3,5-hexatriene, and all-trans-1,3,5,7-octatetraene. The ordering of the excited states is correctly reproduced by the DFT-SSMRPT calculations. The relative ordering of low-lying excited 1Bu and 1Ag states alters when the length of the polyene changes. The results match reasonably well with the literature including experimental and best theoretical findings. The accuracy of the method is sufficient to discern the energy gap between the close low-lying singlet and triplet states. The DFT-SSMRPT appears as an affordable computational ab initio avenue for a qualitatively correct description of excitation energies. [ABSTRACT FROM AUTHOR]

We report the Auger and Coster-Kronig transition energies (related to double ionization potentials) of noble gas elements obtained using the Fock-space multireference coupled cluster (FSMRCC) method with relativistic spinors. The resulting Auger and Coster-Kronig lines are found to be in agreement with the experimental data and with other reference theoretical estimates. To the best of our knowledge, no prior report of relativistic calculations is available for Auger transition energies at the FSMRCC level of theory. The ionization potentials resulted from this method with no extra cost are also found to be in agreement with experiment, particularly the outer-valence ones. Interestingly, the FSMRCC and the multiconfiguration Dirac-Fock calculations exhibit an inversion in the 3P energy levels of the xenon atom for N4,5–O23O23 Auger transitions, where the 3Pj state energies appear in the order J = 1, 0, 2, a feature which can be verified experimentally. [ABSTRACT FROM AUTHOR]

Adaptation of improved virtual orbital complete active space configuration interaction functions in state-specific multireference perturbation theory motivated by the Brillouin-Wigner perturbation scheme using Møller-Plesset multipartitioning is examined. The method, denoted as IVO-BWMRPT, focuses on only the root of principal interest at a time using single-root parameterization of Jeziorski-Monkhorst ansatz within the frame of an effective Hamiltonian. This approach yields size-extensive energy and avoids intruder-state problems in a natural manner. It allows relaxation of the reference space wave function in the presence of the perturbation which produces an important differential effect on the energy and cannot be neglected for quasidegenerate electronic states. The method has been tested against nontrivial situations such as the Be + H2 insertion profile along with the energy surfaces of FH and X2 (X = F, Cl, and Br), in which conventional single-reference methods generally fail, exhibiting very encouraging findings. We also consider the energy surfaces of ethylene (by breaking the π bond as well as the CC bond) and for the twisting of tetramethyleneethane. IVO-BWMRPT represents a rather balanced protocol for the description of molecules at a wide range of geometries, including stretched or dissociating bonds. Close agreement of our estimates with the reference values provides a useful measure for the success of the IVO-BWMRPT method to treat strongly correlated systems. Our results for TME show that the singlet state always lies below the triplet state for different conformations. The IVO-BWMRPT furnishes a compact and correct representation of the MR-wave function, and hence, a large variety of quasidegenerate situations can be accommodated within the method. [ABSTRACT FROM AUTHOR]

A relativistic study of spectroscopic properties of the endohedral fullerenes Ng@C60q (where Ng = He, Ne and q=0,±1,±2 are the charges) associated with the C60 molecule has been done using the equation of motion coupled cluster (EOM-CC) methodology. Specific properties estimated are the transition energies, dipole oscillator strengths, and transition probabilities for the low-lying excitations 1s²(1S0) → 1snp (¹P1) (n = 2, 3, 4) for He@C60q and 1s²2s²2p6 (1S0) → 1s²2s²2p5ns/nd (¹P1) (n = 3, 4) for Ne@C60q, which have been compared with those for the isolated atom to depict the confinement effect of the host molecule on the encapsulated atom. This is accomplished by introducing an effective potential to the atomic Hamiltonian induced by the fullerene moiety and its charge. The EOM-CC results have been compared with those estimated with the random phase approximation (and configuration interaction singles) to understand the effect of electron correlation under such confinement. The systematic and interesting behavior of the properties is highlighted indicating the effect of fullerene cage potential on the redistribution of electron density of the guest atom. [ABSTRACT FROM AUTHOR]

The state-specific multireference perturbation theory (SSMRPT) with an improved virtual orbital complete active space configuration interaction (IVO-CASCI) reference function [called as IVOSSMRPT] is used to investigate the energy surface, geometrical parameters, molecular properties of spectroscopic interest for the systems/situations [such as BeH2, BeCH2, MgCH2, Si2H4, unimolecular dissociation of H2CO, and intramolecular reaction pathways of 1,3-butadiene] where the effect of quasidegeneracy cannot be neglected. The merit of using the IVO-CASCI rather than complete active space self-consistent field (CASSCF) is that it is free from iterations beyond those in the initial SCF calculation and the convergence difficulties that plague CASSCF calculations with increasing size of the CAS. While IVO-CASCI describes the non-dynamical correlation, the SSMRPT scheme is a good second-order perturbative approximation to account for the rest of the correlation energy. Our IVO-SSMRPT method is instrumental in avoiding intruder states in an size-extensive manner and allows the revision of the content of wave function in the model space. It can treat model as well as real systems with predictive accuracy, as is evident from the fairly nice accordance between our estimates, and high-level theoretical results. Our estimates also corroborate well with some experimental findings. [ABSTRACT FROM AUTHOR]

A four-component (4c) relativistic state specific multireference coupled cluster (4c-SSMRCC) method has been developed and applied to compute the ground state spectroscopic constants of Ag2, Cu2, Au2, and I2. The reference functions used in these calculations are obtained using computationally inexpensive improved virtual orbital-complete active space configuration interaction scheme. Rigorous size-extensivity and insensitivity towards the intruder state problem make our method an interesting choice for the calculation of the dissociation energy surface. To the best of our knowledge, this study is the first implementation of the SSMRCC within the relativistic framework. The overall agreement of our results, employing the smallest model space, with both theoretical and experimental reference values indicates that the 4c-SSMRCC method can be fruitfully used to describe electronic structures and associated properties of systems containing heavy elements. We observe a relativistic bond stabilization for the coinage metal dimers while the I-I bond is weakened by the relativistic effects. [ABSTRACT FROM AUTHOR]

The equation of motion coupled cluster methodology within relativistic framework has been applied to analyze the electron correlation effects on the low lying dipole allowed excited states of Ne and Al3+ under classical and quantum plasma environments. The effect of confinement due to classical plasma has been incorporated through screened Coulomb potential, while that of quantum plasma has been treated by exponential cosine screened Coulomb potential. The confined structural properties investigated are the depression of ionization potential, low lying excitation energies (dipole allowed), oscillator strengths, transition probabilities, and frequency dependent polarizabilities under systematic variation of the plasma-atom coupling strength determined through the screening parameter. Specific atomic systems are chosen for their astrophysical importance and availability of experimental data related to laboratory plasma with special reference to Al3+ ion. Here, we consider 1 s 2 2 s 2 2 p 6 ( 1 S 0 ) → 1 s 2 2 s 2 2 p 5 n s / n d ( 1 P 1 ) ( n = 3 , 4 ) dipole allowed transitions of Ne and Al3+. Results for the free (isolated) atomic systems agree well with those available in the literature. Spectroscopic properties under confinement show systematic and interesting pattern with respect to plasma screening parameter. [ABSTRACT FROM AUTHOR]

To obtain even qualitatively correct results of potential energy surfaces (PESs) of the ground and two lowest-lying excited singlet states of C2is a difficult task due to the strong geometry-dependent closeness of these PESs leading to real and avoided crossings. Our IVO-SSMRPT method in which the nondynamical correlation is simplified with IVO-CASCI has been applied to these states. IVO-SSMRPT provides comparable results with reference theoretical and experimental data indicating all components of the approach work in harmony for a correct representation of the surfaces including the locations of the challenging crossing points between the states. [ABSTRACT FROM PUBLISHER]

Adaptation of improved virtual orbitals (IVOs) in state-specific multireference perturbation theory using Møller-Plesset multipartitioning of the Hamiltonian (IVO-SSMRPT) is examined in which the IVO-complete active space configuration interaction (CASCI) is used as an inexpensive alternative to the more involved CAS-self-consistent field (CASSCF) orbitals. Unlike the CASSCF approach, IVO-CASCI does not bear tedious and costly iterations beyond those in the initial SCF calculation. The IVO-SSMRPT is intruder-free, and explicitly size-extensive. In the present preliminary study, the IVO-SSMRPT method which relies on a small reference space is applied to study potential energy surfaces (PES) of the ground state of challenging, multiconfigurational F2, Be2, and N2 molecules. These systems provide a serious challenge to any ab initio methodology due to the presence of an intricate interplay of nondynamical and dynamical correlations to the entire PES. The quality of the computed PES has been judged by extracting spectroscopic parameters and vibrational levels. The reported results illustrate that the IVO-SSMRPT method has a potential to yield accuracies as good as the CASSCF-SSMRPT one with reduced computational labor. Even with small reference spaces, our estimates demonstrate a good agreement with the available experimental values, and some benchmark computations. The blend of accuracy and low computational cost of IVO-SSMRPT should deserve future attention for the accurate treatment of electronic states of small to large molecular systems for which the wavefunction is characterized by various configurations. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Adaptation of improved virtual orbitals (IVOs) in state-specific multireference perturbation theory using Møller-Plesset multipartitioning of the Hamiltonian (IVO-SSMRPT) is examined in which the IVO-complete active space configuration interaction (CASCI) is used as an inexpensive alternative to the more involved CAS-self-consistent field (CASSCF) orbitals. Unlike the CASSCF approach, IVO-CASCI does not bear tedious and costly iterations beyond those in the initial SCF calculation. The IVO-SSMRPT is intruder-free, and explicitly size-extensive. In the present preliminary study, the IVO-SSMRPT method which relies on a small reference space is applied to study potential energy surfaces (PES) of the ground state of challenging, multiconfigurational F2, Be2, and N2 molecules. These systems provide a serious challenge to any ab initio methodology due to the presence of an intricate interplay of nondynamical and dynamical correlations to the entire PES. The quality of the computed PES has been judged by extracting spectroscopic parameters and vibrational levels. The reported results illustrate that the IVO-SSMRPT method has a potential to yield accuracies as good as the CASSCF-SSMRPT one with reduced computational labor. Even with small reference spaces, our estimates demonstrate a good agreement with the available experimental values, and some benchmark computations. The blend of accuracy and low computational cost of IVO-SSMRPT should deserve future attention for the accurate treatment of electronic states of small to large molecular systems for which the wavefunction is characterized by various configurations. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The relative stabilities ofcis-andtrans-isomers of 1,2-difluoroethylene and 1,2-difluorodiazene have been studied via the state-specific multireference coupled cluster (SS-MRCC) method and its perturbative counterpart through the computation of the optimised structures and corresponding energies. Despite the existence of severalcis-destabilising mechanisms, present calculations reveal the energetic preference of thecis-isomer (thecis-effect) for the systems considered here. Differences in structural parameters and vibrational frequencies amongcis-andtrans-isomers have been discussed. Very good agreement of our estimates has been found with the benchmark theoretical and experimental results. The SS-MRCC methods produce a smooth and consistent behaviour across the entire torsional surface for thecis–transisomerisation indicating that the method has sufficient flexibility to model large changes in electronic structure that accompany chemical changes. [ABSTRACT FROM AUTHOR]

We have computed the potential energy surfaces (PESs) of F2, Cl2, Br2, and I2using the size-extensive state specific multireference coupled cluster (SS-MRCC) method. The MR character of the system considered here at large distances and the presence of low-lying intruder states are known to be the major causes of incorrect or inaccurate predictions of the PES. The SS-MRCC theory is tailored to treat degeneracies of varying extent while bypassing the intruder problem. The quality of the computed PES has been gauged by computing spectroscopic constants. The calculated properties show a good agreement with available experimental data and the errors in the calculated molecular properties compare favourably with the most elaborate current-generation calculations of the literature. The accuracy of the computed PES of F2is such that it has been proved to calculate the vibrational spectrum of the 22 levels with a minimum and maximum absolute deviation of 2 and 57 cm−1, respectively, from the experimental values. The highly satisfactory performance of the SS-MRCC method, vis-a-vis the other sophisticated methods, in describing the vibrational levels is noticeable for one of the more difficult systems such as F2clearly indicates that the present method is reliable in studying the vibrational energy levels. [ABSTRACT FROM PUBLISHER]

IVO‐SSMRPT is an affordable and accurate type of state‐specific multireference perturbation (SSMRPT) theory that adds dynamic correlation energy to improved virtual orbital (IVO) complete active space configuration interaction (CASCI) wave functions using a single‐root parametrization of multi‐root Hilbert‐space ansatz. We applied it to many chemically important di‐ and tri‐radicals to analyze the geometries and electronic properties of spectroscopic interest for both closed‐ and open‐shell singlet‐ and nonsinglet ground as well as excited states. We observed that IVO‐SSMRPT identifies optimized geometries, splitting between multiplets and frequencies for several radicals that are similar to those displayed by current generation state‐of‐the‐art methods but with admiringly decreased computational effort. This study illustrates the importance of having an accurate treatment of both nondynamical and dynamical correlation effects when examining multiradical species. Chemically and spectroscopically relevant answers can be obtained using our computationally tractable method. Our method will be a serviceable avenue for portraying open‐shell interactions in other radicals. Interest in organic radicals is fuelled by their potential role in designing organic materials and complex electronic structure. A multireference perturbative method with improved virtual orbitals has been used as an affordable, efficient and robust device for studying radicals. The method is tailored to tackle open‐ and closed‐shell states with the same accuracy. An intricate balance between different correlation effects is tangled in the accurate description of multiple states and their gaps. [ABSTRACT FROM AUTHOR]

Adaptation of improved virtual orbitals (IVOs) in state-specific multireference perturbation theory using Møller-Plesset multipartitioning of the Hamiltonian (IVO-SSMRPT) is examined in which the IVO-complete active space configuration interaction (CASCI) is used as an inexpensive alternative to the more involved CAS-self-consistent field (CASSCF) orbitals. Unlike the CASSCF approach, IVO-CASCI does not bear tedious and costly iterations beyond those in the initial SCF calculation. The IVO-SSMRPT is intruder-free, and explicitly size-extensive. In the present preliminary study, the IVO-SSMRPT method which relies on a small reference space is applied to study potential energy surfaces (PES) of the ground state of challenging, multiconfigurational F2 , Be2 , and N2 molecules. These systems provide a serious challenge to any ab initio methodology due to the presence of an intricate interplay of nondynamical and dynamical correlations to the entire PES. The quality of the computed PES has been judged by extracting spectroscopic parameters and vibrational levels. The reported results illustrate that the IVO-SSMRPT method has a potential to yield accuracies as good as the CASSCF-SSMRPT one with reduced computational labor. Even with small reference spaces, our estimates demonstrate a good agreement with the available experimental values, and some benchmark computations. The blend of accuracy and low computational cost of IVO-SSMRPT should deserve future attention for the accurate treatment of electronic states of small to large molecular systems for which the wavefunction is characterized by various configurations., (© 2015 Wiley Periodicals, Inc.)