Large-scale parallel configuration interaction. I. Nonrelativistic and scalar-relativistic general active space implementation with application to (Rb–Ba)+

Structure and Excitation Spectra of Third-Row Transition Metal Hexafluorides Based on Multi-Reference Exact Two-Component Theory

The structures and some vertical excitation energies of third-row transition metal hexafluorides (MF6, M = Re, Os, Ir, Pt, Au, Hg) were calculated using the generalized-active-space configuration interaction (GASCI) theory based on the exact two-component (X2C) Hamiltonian. The spin-orbit coupling (SOC) was included at the Hartree-Fock level, enabling us to analyze the SOC at the orbital level (spinor-representation). The excitation spectra were assigned based on the double group, a relativistic group theory applicable to states with the SOC. This study provides a fundamental understanding of the ligand field splitting, including the SOC, that is useful for the photochemistry and spin chemistry involving heavy elements.

Cholesky Decomposition in Spin-Free Dirac-Coulomb Coupled-Cluster Calculations

We present an implementation for the use of Cholesky decomposition (CD) of two-electron integrals within the spin-free Dirac-Coulomb (SFDC) scheme that enables to perform high-accuracy coupled-cluster (CC) calculations at costs almost comparable to those of their nonrelativistic counterparts. While for nonrelativistic CC calculations, atomic-orbital (AO)-based algorithms, due to their significantly reduced disk-space requirements, are the key to efficient large-scale computations, such algorithms are less advantageous in the SFDC case due to their increased computational cost in that case. Here, molecular-orbital (MO)-based algorithms exploiting the CD of the two-electron integrals allow us to reduce disk-space requirements and lead to computational cost in the CC step that is more or less the same as in the nonrelativistic case. The only remaining overhead in a CD-SFDC-CC calculation is due to the need to compute additional two-electron integrals, the somewhat higher cost of the Hartree-Fock calculation in the SFDC case, and additional cost in the transformation of the Cholesky vectors from the AO to the MO representation. However, these additional costs typically amount to less than 5-15% of the total wall time and are thus acceptable. We illustrate the efficiency of our CD scheme for SFDC-CC calculations on a series of illustrative calculations for the X(CO)4 molecules with X = Ni, Pd, Pt.

On the third-order nonlinear optical responses of cis and trans stilbenes - a quantum chemistry investigation

The second hyperpolarizabilities (γ) of the stilbene molecular switch in its trans and cis forms have been calculated using quantum chemistry methods to address their third-order nonlinear optical contrasts, to assess the reliability of lower-cost DFT methods, and to make comparisons with experiments. First, the reference CCSD(T) method shows that trans-stilbene presents a γ‖ value twice larger than its cis isomer (its γTHS value is 2.7 times larger). Among more cost-effective methods, reliable results are obtained at MP2 as well as with DFT, provided the CAM-B3LYP or ωB97X-D XCFs are employed. Supplementary DFT calculations have investigated the relationships between the accuracy of the exchange-correlation functionals, the fulfillment of Koopmans' theorem, and the delocalization error, and they demonstrated that satisfying Koopmans' theorem is not the condition for the best accuracy but that functionals with small delocalization errors are generally efficient. Using the selected CAM-B3LYP, large γ enhancements by about 70% (trans-stilbene) and 50% (cis-stilbene) have been evidenced when accounting for solvent effects using an implicit solvation model (IEFPCM), even for apolar solvents. Then, the frequency dispersion of the γ responses has been described using Bishop polynomial expansions, allowing comparisons with a broad set of experimental data. To a certain extent, no systematic agreement between the calculations and the measured values was found. On the one hand, the agreement is satisfactory for the γ(-ω;ω,-ω,ω) quantities, provided that the dominant vibrational contribution is taken into account. On the other hand, the agreement is poor for the γ(-2ω;ω,ω,0) and γ(-3ω;ω,ω,ω) quantities, while some inconsistencies between experimental values are also highlighted.

Bond Dissociation Energies in Heavy Element Chalcogen and Halogen Small Molecules

Thermodynamic properties including bond dissociation energies (BDEs), heats of formation, and gas-phase acidities for the hydrides and dimers of chalcogens and halogens, H₂Y, HX, Y₂, and X₂ for Y = Se, Te, and At and X = Br, I, and At, have been predicted using the Feller-Peterson-Dixon composite-correlated molecular orbital theory approach. A full four-component CCSD(T) approach was used to calculate the spin-orbit effects on thermodynamic properties, except for Se₂, where the AoC-DHF value was used due to strong multireference effects in Se₂ for the SO calculations. The calculated results show that the At₂ BDE is quite small, 19.5 kcal/mol, with much of the low bond energy due to spin-orbit effects. H₂Po is not predicted to be stable to dehydrogenation to Po + H₂ in terms of the free energy at 298 K. In the gas phase, HAt is predicted to be a stronger acid than H₂SO₄. The current results provide insights into potential difficulties in the actual experimental observation of such species for heavy elements.

Portably parallel construction of a configuration-interaction wave function from a matrix-product state using the Charm++ framework

The construction of configuration-interaction (CI) expansions from a matrix product state (MPS) involves numerous matrix operations and the skillful sampling of important configurations in a large Hilbert space. In this work, we present an efficient procedure for constructing CI expansions from MPS employing the parallel object-oriented Charm++ programming framework, upon which automatic load-balancing and object migrating facilities can be employed. This procedure was employed in the MPS-to-CI utility (Moritz et al., J. Chem. Phys. 2007, 126, 224109), the sampling-reconstructed complete active-space algorithm (SR-CAS, Boguslawski et al., J. Chem. Phys. 2011, 134, 224101), and the entanglement-driven genetic algorithm (EDGA, Luo et al., J. Chem. Theory Comput. 2017, 13, 4699). It enhances productivity and allows the sampling programs to evolve to their population-expansion versions, for example, EDGA with population expansion (PE-EDGA). Further, examples of 1,2-dioxetanone and firefly dioxetanone anion (FDO^- ) molecules demonstrated the following: (a) parallel efficiencies can be persistently improved by simply by increasing the proportions of the asynchronous executions and (b) a sampled CAS-type CI wave function of a bi-radical-state FDO^- molecule utilizing the full valence (30e,26o) active space can be constructed within a few hours with using thousands of cores.

The DIRAC code for relativistic molecular calculations

DIRAC is a freely distributed general-purpose program system for one-, two-, and four-component relativistic molecular calculations at the level of Hartree-Fock, Kohn-Sham (including range-separated theory), multiconfigurational self-consistent-field, multireference configuration interaction, electron propagator, and various flavors of coupled cluster theory. At the self-consistent-field level, a highly original scheme, based on quaternion algebra, is implemented for the treatment of both spatial and time reversal symmetry. DIRAC features a very general module for the calculation of molecular properties that to a large extent may be defined by the user and further analyzed through a powerful visualization module. It allows for the inclusion of environmental effects through three different classes of increasingly sophisticated embedding approaches: the implicit solvation polarizable continuum model, the explicit polarizable embedding model, and the frozen density embedding model.

Electronic Structure Predictions of the Energetic Properties of Tellurium Fluorides

The heats of formation, bond dissociation energies (BDEs), fluoride affinities (FA), fluorocation affinities (FCA), electron affinities (EA), and ionization energies (IP) of TeF _n ( n = 1-6) have been predicted using the Feller-Peterson-Dixon (FPD) approach. To benchmark the approach, the bond dissociation energies of Te₂ and TeO, the heats of formation of Te₂, TeH₂, TeO, and TeO₂, and the electron affinity for TeO and TeO₂ were calculated as there are experimental thermodynamic data available for these tellurium compounds, which allow confirmation of the heat of formation of Te gas as Δ H_f,0K(Te) = 50.7 ± 0.6 kcal/mol. Spin-orbit corrections are required for good results and cannot be ignored. A comparison among fluoride affinities, fluorocation affinities, electron affinities, and ionization energies of TeF _n and SeF _n is reported.

Guided ion beam and theoretical studies of the bond energy of SmS. J Chem Phys 2017;147:214307. [PMID: 29221388 DOI: 10.1063/1.5009916]

Previous work has shown that atomic samarium cations react with carbonyl sulfide to form SmS⁺ + CO in an exothermic and barrierless process. To characterize this reaction further, the bond energy of SmS⁺ is determined in the present study using guided ion beam tandem mass spectrometry. Reactions of SmS⁺ with Xe, CO, and O₂ are examined. Results for collision-induced dissociation processes with all three molecules along with the endothermicity of the SmS⁺ + CO → Sm⁺ + COS exchange reaction are combined to yield D₀(Sm⁺-S) = 3.37 ± 0.20 eV. The CO and O₂ reactions also yield a SmSO⁺ product, with measured endothermicities that indicate D₀(SSm⁺-O) = 3.73 ± 0.16 eV and D₀(OSm⁺-S) = 1.38 ± 0.27 eV. The SmS⁺ bond energy is compared with theoretical values characterized at several levels of theory, including CCSD(T) complete basis set extrapolations using all-electron basis sets. Multireference configuration interaction calculations with explicit spin-orbit calculations along with composite thermochemistry using the Feller-Peterson-Dixon method and all-electron basis sets were also explored for SmS⁺, and for comparison, SmO, SmO⁺, and EuO.

Stability and spectral properties of the dication Ne2+2

Two different types of metastable states in Ne2+2 are predicted and possible decay transitions as well as the ensuing lifetimes and intensity distributions are studied.

Relativistic configuration interaction calculation on the ground and excited states of iridium monoxide

We present the fully relativistic multi-reference configuration interaction calculations of the ground and low-lying excited electronic states of IrO for individual spin-orbit component. The lowest-lying state is calculated for Ω = 1/2, 3/2, 5/2, and 7/2 in order to clarify the ground state of IrO. Our calculation suggests that the ground state is of Ω = 1/2, which is highly mixed with (4)Σ(-) and (2)Π states in Λ - S notation. The two low-lying states 5/2 and 7/2 are nearly degenerate with the ground state and locate only 234 and 260 cm(-1) above, respectively. The equilibrium bond length 1.712 Å and the harmonic vibrational frequency 903 cm(-1) of the 5/2 state are close to the experimental measurement of 1.724 Å and 909 cm(-1), which suggests that the 5/2 state should be the low-lying state that contributes to the experimental spectra. Moreover, the electronic states that give rise to the observed transition bands are assigned for Ω = 5/2 and 7/2 in terms of the obtained excited energies and oscillator strengths.

Electronic spectra of DyF studied by four-component relativistic configuration interaction methods

The electronic states of the DyF molecule below 3.0 eV are studied using 4-component relativistic CI methods. Spinors generated by the average-of-configuration Hartree-Fock method with the Dirac-Coulomb Hamiltonian were used in CI calculations by the KRCI (Kramers-restricted configuration interaction) program. The CI reference space was generated by distributing 11 electrons among the 11 Kramers pairs composed mainly of Dy [4f], [6s], [6p] atomic spinors, and double excitations are allowed from this space to the virtual molecular spinors. The CI calculations indicate that the ground state has the dominant configuration (4f(9))(6s(2))(Ω = 7.5). Above this ground state, 4 low-lying excited states (Ω = 8.5, 7.5, 7.5, 7.5) are found with dominant configurations (4f(10))(6s). These results are consistent with the experimental studies of McCarthy et al. Above these 5 states, 2 states were observed at T0 = 2.39 eV, 2.52 eV by McCarthy et al. and were named as [19.3]8.5 and [20.3]8.5. McCarthy et al. proposed that both states have dominant configurations (4f(9))(6s)(6p), but these configurations are not consistent with the large Re's (∼3.9 a.u.) estimated from the observed rotational constants. The present CI calculations provide near-degenerate states of (4f(10))(6p3/2,1/2), (4f(10))(6p3/2,3/2), and (4f(9))(6s)(6p3/2,1/2) at around 3 eV. The former two states have larger Re (3.88 a.u.) than the third, so that it is reasonable to assign (4f(10))(6p3/2,1/2) to [19.3]8.5 and (4f(10))(6p3/2,3/2) to [20.3]8.5.

Theoretical study on the ground state of the polar alkali-metal-barium molecules: potential energy curve and permanent dipole moment

In this paper, we systematically investigate the electronic structure for the (2)Σ(+) ground state of the polar alkali-metal-alkaline-earth-metal molecules BaAlk (Alk = Li, Na, K, Rb, and Cs). Potential energy curves and permanent dipole moments (PDMs) are determined using power quantum chemistry complete active space self-consistent field and multi-reference configuration interaction methods. Basic spectroscopic constants are derived from ro-vibrational bound state calculation. From the calculations, it is shown that BaK, BaRb, and BaCs molecules have moderate values of PDM at the equilibrium bond distance (BaK:1.62 D, BaRb:3.32 D, and BaCs:4.02 D). Besides, the equilibrium bond length (4.93 Å and 5.19 Å) and dissociation energy (0.1825 eV and 0.1817 eV) for the BaRb and BaCs are also obtained.

The Dalton quantum chemistry program system

Dalton is a powerful general-purpose program system for the study of molecular electronic structure at the Hartree-Fock, Kohn-Sham, multiconfigurational self-consistent-field, Møller-Plesset, configuration-interaction, and coupled-cluster levels of theory. Apart from the total energy, a wide variety of molecular properties may be calculated using these electronic-structure models. Molecular gradients and Hessians are available for geometry optimizations, molecular dynamics, and vibrational studies, whereas magnetic resonance and optical activity can be studied in a gauge-origin-invariant manner. Frequency-dependent molecular properties can be calculated using linear, quadratic, and cubic response theory. A large number of singlet and triplet perturbation operators are available for the study of one-, two-, and three-photon processes. Environmental effects may be included using various dielectric-medium and quantum-mechanics/molecular-mechanics models. Large molecules may be studied using linear-scaling and massively parallel algorithms. Dalton is distributed at no cost from http://www.daltonprogram.org for a number of UNIX platforms.