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A valence-universal multireference coupled cluster (VU-MRCC) theory handles completely general incomplete model spaces, remaining close to the intermediate normalization (IN) condition for OMEGA as much as possible without violating extensivity and without the use of a post facto correction. An effective Hamiltonian preserves the extensivity of the target energies while it is also indicated that IN for OMEGA is a valid size-extensive normalization for quasi-complete model space and isolated incomplete model space.

A coupled electron-pair (CERA) type variant of the state-specific multireference coupled cluster (SS-MRCC) method is presented. The method termed as SS-MRCEPA based on complete active space can handle quasi-degeneracy of varying degrees over a wide range of potential energy curves (PECs), including regions of real or avoided curve-crossing.

The coupled cluster based linear response theory (CCLRT) with four-component relativistic spinors is employed to compute the electric field gradients (EFG) of 35Cl, 79Br, and 127I nuclei. The EFGs resulting from these calculations are combined with experimental nuclear quadrupole coupling constants (NQCC) to determine the nuclear quadrupole moments (NQM), Q of the halide nuclei. Our estimated NQMs [35Cl = -81.12 mb, 79Br = 307.98 mb, and 127I = -688.22 mb] agree well with the new atomic values [35Cl = -81.1(1.2), 79Br = 302(5), and 127I = -680(10) mb] obtained via Fock space multireference coupled cluster method with the Dirac-Coulomb-Breit Hamiltonian. Although our estimated Q(79Br) value deviates from the accepted reference value of 313(3) mb, it agrees well with the recently recommended value, Q(79Br) = 308.7(20) mb. Good agreement with current reference data indicates the accuracy of the proposed value for these halogen nuclei and lends credence to the results obtained via CCLRT approach. The electron affinities yielded by this method with no extra cost are also in good agreement with experimental values, which bolster our belief that the NQMs values for halogen nuclei derived here are reliable. [ABSTRACT FROM AUTHOR]

Escape from a metastable state inthe presence of a high-frequencyfield (where the driving becomes nonadiabatic) underlies a broad rangeof phenomena of physics and chemistry, and thus its understandingis of paramount importance. We study the problem of intermediate-to-high-dampingescape from a metastable state of a dissipative system driven by arapidly oscillating field, one of the most important classes of nonequilibriumsystems, in a broad range of field driving frequencies (ω) andamplitudes (a). We construct a Langevin equationusing quantum gauge transformation in the light of Floquet theoremand exploiting a systematic perturbative expansion in powers of 1/ωusing “Kapitza–Landau time window”. The quantumdynamics in a high-frequency field are found to be described by aneffective time-independent potential. The temperature dependence ofescape rate and the change of its form with varying parameters ofthe field have been analyzed. It may decrease upon increasing thetemperature which is contingent on the effects of intricate interplaybetween external modulation and dissipation. The crossover temperaturebetween tunnelling and thermal hopping increases with an increasein external modulation so that quantum effects in the escape are relevantat higher temperatures. These observations are uncommon and counterintuitiveand, therefore, of considerable interest. Our results might be valuablefor the exploration of the dynamics of cold atoms in electromagneticfields. [ABSTRACT FROM AUTHOR]

Theoretical investigations of thesuperheavy elements (SHEs) are extremely challenging and are oftenthe sole source of useful chemical information. Relativistic Fock-spacemultireference coupled cluster (RFS-MRCC) computations have been carriedout for evaluating the ionization potential (IP), excitation energies(EE), nuclear magnetic hyperfine constant (A), lifetime(τ), and Landé gfactor of singly ionizedeka-lead (Fl II). To judge the accuracy of Fl II results, similarcalculations are performed for Pb II, which shows a nice and consistentagreement with known experimental values. Thus, we believe that ourpredictions for Fl are reliable and useful for the simulation of experimentalbehavior. To the best of our knowledge, no prior theoretical and/orexperimental information is available for A, τ,and g-factor of this SHE. The higher IPs and EEsof Fl II, with respect to Pb II, indicate the former to be more inertand less metallic than Pb. This is contingent on the effects of therelativistic stabilization of the 7s and 7p1/2orbitals.The present analysis demonstrates the influence of higher-body clusteroperators on atomic properties. The close agreement with the experiment(having an estimated error within 1–2%) indicates that theFS-MRCC method is a reliable predictive tool in cases where the experimentalresults are not readily available, such as the SHEs. The remainingsource of error possibly stems out from the omission of the full-blowntriple virtual excitations and the absence of Breit interaction. [ABSTRACT FROM AUTHOR]

The performance of a numerically oriented gradient scheme for the previously introduced second-order state-specific multireference Møller-Plesset perturbation theory (SS-MRMPPT) has been tested to compute certain geometrical parameters (such as bond lengths and angles). Various examples [H2O, O3, N2H2, C2H4, C2H2F2, 1,3-butadiene (C4H6), cyclobutadiene (C4H4), and 2,6-pyridynium cation (C5NH4+)] have been presented to validate the implementation of the SS-MRMPPT gradient approach. To illustrate the reliability of our findings, comparisons have been made with the previously reported theoretical results. The accuracy of our calculations has further been assessed by comparing with the experimental results whenever available. On the basis of the present work, we arrive at the conclusion that the SS-MRMPPT gradient scheme has substantial potential in computing the geometrical parameters for several rather diverse molecular systems, whether charged or neutral and having the closed-shell ground state or being open-shell radicals or biradicals or strongly perturbed by intruders. It is worthwhile to emphasize that the present work represents the first systematic application of the SS-MRMPPT numerical gradient approach. [ABSTRACT FROM AUTHOR]

The ground and low-lying excited state electronic structural properties (such as equilibrium geometries, harmonic frequencies, excitation energies, barrier energy, and so on) of the methylene amidogene radical (H 2 CN) and its anion (H 2 CN - ) have been studied using the CASCI (complete active space configuration interaction) and SSMRPT (state-specific multireference Møller-Plesset perturbation theory) methods with density function theory (DFT) orbitals. Here, the span of the active orbitals have been obtained from Kohn-Sham DFT using B3LYP exchange-correlation functionals in the CASCI (DFT-CASCI) approximation to describe nondynamic correlation associated with electronic degeneracies. The DFT-SSMRPT protocol provides an attractive way to deal with both dynamical and nondynamical correlation effects in strongly correlated systems such as H 2 CN and H 2 CN - . The present work clearly indicates that the electronic absorption band near 35,050 cm -1 corresponds to the B̃ 2 A 1 ← X̃ 2 B 2 transition. DFT-SSMRPT findings are in close agreement with high-level theoretical estimates. It is concluded that the transition at 1725 cm -1 could be due to the CN stretching of the trans -HCNH isomer which is originally assigned to the CN stretch of H 2 CN in the experiment. The present results confirm most of the previous vibrational assignments. It is not possible to definitively assign a transition to the 35,600 cm -1 band with the present estimations, suggesting further experiment is urgently called for.

An improved virtual orbital complete active space configuration interaction reference function-based multireference Brillouin-Wigner perturbation approach (IVO-BWMRPT) that avoids the numerical divergence because of the intruder problem by focusing on obtaining a single root of the many-body Hamiltonian is used to compute dissociation energy surfaces and spectroscopic constants of C 2 , N 2 , and CN radicals. For such correlated molecules, the computational demand to delineate the wavefunction exactly is quite challenging. The IVO-BWMRPT method, a convenient and effective way to handle quasidegenerate situations, accurately captures different correlations and is capable of treating the variation of multiconfigurational nature of wave functions that occur during the multiple-bond breaking processes. Spectroscopic constants extracted from the computed surface are in good agreement with experimental or benchmark results, indicating that the components of the IVO-BWMRPT scheme must perform in harmony for providing a well-behaved and consistent description of all computed bond lengths.

Recently a state-specific multireference perturbation theory (SSMRPT) with an improved virtual orbitals complete active space configuration interaction (IVO-CASCI) reference function has been proposed for treating electronic structures of radicals such as methylene, m-benzyne, pyridyne, and pyridynium cation. This new development in MRPT, termed as IVO-SSMRPT, ensures that it is able to describe the structure of radicaloids with reasonable accuracy even with small reference spaces. IVO-SSMRPT is also capable of predicting the correct ordering of the lowest singlet-triplet gaps. Investigation of the first three electronic states of the oxygen molecule has also been used for rating our method. The agreement of our estimates with the available far more expensive benchmark state-of-the-art ab initio calculations is creditable. The IVO-SSMRPT method provides an effective avenue with manageable cost/accuracy ratio for accurately dealing with radicaloid systems possessing varying degrees of quasidegeneracy.

The coupled cluster based linear response theory (CCLRT) with four-component relativistic spinors is employed to compute the electric field gradients (EFG) of (35)Cl, (79)Br, and (127)I nuclei. The EFGs resulting from these calculations are combined with experimental nuclear quadrupole coupling constants (NQCC) to determine the nuclear quadrupole moments (NQM), Q of the halide nuclei. Our estimated NQMs [(35)Cl = -81.12 mb, (79)Br = 307.98 mb, and (127)I = -688.22 mb] agree well with the new atomic values [(35)Cl = -81.1(1.2), (79)Br = 302(5), and (127)I = -680(10) mb] obtained via Fock space multireference coupled cluster method with the Dirac-Coulomb-Breit Hamiltonian. Although our estimated Q((79)Br) value deviates from the accepted reference value of 313(3) mb, it agrees well with the recently recommended value, Q((79)Br) = 308.7(20) mb. Good agreement with current reference data indicates the accuracy of the proposed value for these halogen nuclei and lends credence to the results obtained via CCLRT approach. The electron affinities yielded by this method with no extra cost are also in good agreement with experimental values, which bolster our belief that the NQMs values for halogen nuclei derived here are reliable.

Optimized geometries are evaluated for the ground and low lying excited states of cis-stilbene, trans-stilbene, and 4a,4b-dihydrophenanthrene (DHP) from calculations performed with the improved virtual orbital, complete active space configuration interaction (IVO-CASCI) method. The calculations indicate that a nonplanar conformer of trans-stilbene is the most stable among the isomers. The calculated ground and low lying excited state geometries agree well with experiment and with prior theoretical estimates where available. Our IVO-CASCI based multireference Möller-Plesset (MRMP) computations place the (1)B(u) state of trans stilbene to be ∼4.0 eV above the ground X(1)A(g) state, which is in accord with experiment and with earlier theoretical estimates. The 1(1)B(u) state of trans-stilbene can be represented by the highest occupied molecular orbital (HOMO) → lowest unoccupied molecular orbital (LUMO) transition (ionic type) from the ground state, whereas its 2(1)B(u) state is dominated by the HOMO → LUMO+1 and HOMO-1 → LUMO transitions (covalent type). Likewise, the 1(1)B and 2(1)B states of cis-stilbene and DHP are also found to be of ionic and covalent types, respectively.

The improved virtual orbital (IVO) complete active space (CAS) configuration interaction (IVO-CASCI) method is a simplified CAS self-consistent field (SCF), CASSCF, method. Unlike the CASSCF approach, the IVO-CASCI method does not require iterations beyond an initial SCF calculation, rendering the IVO-CASCI scheme computationally more tractable than the CASSCF method and devoid of the convergence problems that sometimes plague CASSCF calculations as the CAS size increases, while retaining all the essential positive benefits of the CASSCF method. Earlier applications demonstrate that the IVO-CASCI energies are at least as accurate as those from the CASSCF and provide the impetus for our recent development of the analytical derivative procedures that are necessary for a wide applicability of the IVO-CASCI approach. Here we test the ability of the analytic energy gradient IVO-CASCI approach (which can treat both closed- and open-shell molecules of arbitrary spin multiplicity) to compute the equilibrium geometries of four organic radicaloid species, namely, (i) the diradicals trimethylenemethane (TMM), 2,6-pyridyne, and the 2,6-pyridynium cation and (ii) a triradical 1,2,3-tridehydrobenzene (TDB), using various basis sets and different choices for the active space. Although these systems and related molecules have fascinated theoretical chemists for many years, their strong multireference character makes their description quite difficult with most standard many-body approaches. Thus, they provide ideal tests to assess the performance of the IVO-CASCI method. The present work demonstrates consistent agreement with far more expensive benchmark state-of-the-art ab initio calculations and thereby indicates that this new gradient method is able to describe the geometries of various radicaloids very accurately, even when small, but qualitatively correct, reference spaces are used. For example, the IVO-CASCI method leads to a monocyclic structure for the 2,6-isomers of the didehydropyridinium (pyridynium) cation and of didehydropyridine (pyridyne), while SCF and single-reference CCSD computations predict an incorrect bicyclic structure. The IVO-CASCI structures and relative stability for the ground (2)A(1) and excited (2)B(2) states of TDB also accord with the experimentally observed IR spectra and with other highly sophisticated theoretical calculations. The blend of accuracy and reduction in computational cost offered by the present IVO-CASCI analytical gradient method clearly demonstrates that the method provides a practical avenue for studying the geometries of various radicaloid species of different levels of complexity.

The size-extensive second-order state-specific (or single root) multireference (MR) perturbation theory (SS-MRPT) in the Brillouin-Wigner (BW) form using Mϕller-Plesset perturbative evaluations of orders up to 2 [termed as SS-MRMPPT(BW)] presents a viable, as well as promising, approach to include both nondynamic and dynamic correlations in the study of the bond-stretching (in multireference/quasidegenerate situations) of molecular species with a manageable cost/accuracy ratio. It combines numerical stability in the presence of an intruder state problem with strict size consistency (when localized orbitals are used). In this paper, the SS-MRMPPT(BW) method has been shown to properly break the bonds (in the ground state) of several diatomic molecules (such as F2, Cl2 and Br2, and BH) that have posed a severe challenge to any many-body theoretical approach due to the presence of quasidegeneracy of varying degrees in the ground state. A comparison of the resulting potentials with the various theoretical results reveals that the method represents a valuable tool that is capable of properly accounting even for very strong quasidegeneracies, while also performing well in nondegenerate situations. In this work, we have also calculated spectroscopic constants (such as equilibrium bond lengths, vibrational frequencies, and dissociation energies) of the ground state of these molecular systems. The SS-MRMPPT spectroscopic constants are compared with the most accurate available ab initio calculations and other theoretical estimates of previous works to calibrate the efficacy of the method. For the sake of completeness, we also compare the computed spectroscopic constants with the experimental observations. The accuracy of computed spectroscopic parameters appears to be rather consistent over a multitude of systems for various basis sets. The SS-MRMPPT enables quantitatively accurate and computationally affordable analysis of potential energy surfaces and spectroscopic constants of various multireference systems in the ground state. It is particularly visible for spectroscopic parameters and nonparallelism error (NPE) calculations. The calculations further reveal that the SS-MRMPPT(BW) method compared to its Rayleigh-Schrödinger counterpart [SS-MRMPPT(RS)] provides a more accurate and consistent solution for the whole dissociation path and spectroscopic constants.