Computation of potential energy surfaces with the multireference correlation consistent composite approach

DMRG-Tailored Coupled Cluster Method in the 4c-Relativistic Domain: General Implementation and Application to the NUHFI and NUF3 Molecules

Heavy atom compounds represent a challenge for computational chemistry due to the need for simultaneous treatment of relativistic and correlation effects. Often such systems also exhibit strong correlation, which hampers the application of perturbation theory or single-reference coupled cluster (CC) methods. As a viable alternative, we have proposed externally correcting the CC method using the density matrix renormalization group (DMRG) wave functions, yielding the DMRG-tailored CC method. In a previous paper [J. Chem. Phys. 2020, 152, 174107], we reported a first implementation of this method in the relativistic context, which was restricted to molecules with real double group symmetry. In this work, we present a fully general implementation of the method, covering complex and quaternion double groups as well. The 4c-TCC method thus becomes applicable to polyatomic molecules, including heavy atoms. For the assessment of the method, we performed calculations of the chiral uranium compound NUHFI, which was previously studied in the context of the enhancement of parity violation effects. In particular, we performed calculations of a cut of the potential energy surface of this molecule along the stretching of the N-U bond, where the system exhibits strong multireference character. Since there are no experimental data for NUHFI, we have performed also an analogous study of the (more symmetric) NUF3 molecule, where the vibrational frequency of the N-U bond can be compared with spectroscopic data.

PDG: A Composite Method Based on the Resolution of the Identity

The Gaussian-3 (G3) composite approach for thermochemical properties is revisited in light of the enhanced computational efficiency and reduced memory costs by applying the resolution-of-the-identity (RI) approximation for two-electron repulsion integrals (ERIs) to the computationally demanding component methods in the G3 model: the energy and gradient computations via the second-order Møller-Plesset perturbation theory (MP2) and the energy computations using the coupled-cluster singles-doubles method augmented with noniterative triples corrections [CCSD(T)]. Efficient implementation of the RI-based methods is achieved by employing a hybrid distributed/shared memory model based on MPI and OpenMP. The new variant of the G3 composite approach based on the RI approximation is termed the RI-G3 scheme, or alternatively the PDG method. The accuracy of the new RI-G3/PDG scheme is compared to the "standard" G3 composite approach that employs the memory-expensive four-center ERIs in the MP2 and CCSD(T) calculations. Taking the computation of the heats of formation of the closed-shell molecules in the G3/99 test set as a test case, it is demonstrated that the RI approximation introduces negligible changes to the mean absolute errors relative to the standard G3 model (less than 0.1 kcal/mol), while the standard deviations remain unaltered. The efficiency and memory requirements for the RI-MP2 and RI-CCSD(T) methods are compared to the standard MP2 and CCSD(T) approaches, respectively. The hybrid MPI/OpenMP-based RI-MP2 energy plus gradient computation is found to attain a 7.5× speedup over the standard MP2 calculations. For the most demanding CCSD(T) calculations, the application of the RI approximation is found to nearly halve the memory demand, confer about a 4-5× speedup for the CCSD iterations, and reduce the computational time for the compute-intensive triples correction step by several hours.

Multiconfiguration Pair-Density Functional Theory

Kohn-Sham density functional theory with the available exchange-correlation functionals is less accurate for strongly correlated systems, which require a multiconfigurational description as a zero-order function, than for weakly correlated systems, and available functionals of the spin densities do not accurately predict energies for many strongly correlated systems when one uses multiconfigurational wave functions with spin symmetry. Furthermore, adding a correlation functional to a multiconfigurational reference energy can lead to double counting of electron correlation. Multiconfiguration pair-density functional theory (MC-PDFT) overcomes both obstacles, the second by calculating the quantum mechanical part of the electronic energy entirely by a functional, and the first by using a functional of the total density and the on-top pair density rather than the spin densities. This allows one to calculate the energy of strongly correlated systems efficiently with a pair-density functional and a suitable multiconfigurational reference function. This article reviews MC-PDFT and related background information.

DFT assessment of the spectroscopic constants and absorption spectra of neutral and charged diatomic species of group 11 and 14 elements

The spectroscopic constants and absorption spectra of neutral and charged diatomic molecules of group 11 and 14 elements formulated as [M(2)](+/0/-) (M = Cu, Ag, Au), and [E(2)](+/0/-) (E = C, Si, Ge, Sn, Pb) have been calculated at the PBE0/Def2-QZVPP level of theory. The electronic and bonding properties of the diatomics have been analyzed by natural bond orbital analysis approach and topology analysis by the atoms in molecules method. Particular emphasis was given on the absorption spectra of the diatomic species, which were simulated by time-dependent density functional theory calculations employing the hybrid Coulomb-attenuating CAM-B3LYP density functional. The simulated absorption spectra of the [M(2)](+/0/-) (M = Cu, Ag, Au) and [E(2)](+/0/-) (E = C, Si, Ge, Sn, Pb) species are in close resemblance with the experimentally observed spectra whenever available. The neutral M(2) and E(2) diatomics strongly absorb in the ultraviolet region, given rise to UVC, UVA and in a few cases UVB absorptions. In a few cases, weak absorbion bands also occur in the visible region. The absorption bands have thoroughly been analyzed and assignments of the contributing principal electronic transitions associated to individual excitations have been made.

Assessment of theoretical methods for the study of hydrogen abstraction kinetics of global warming gas species during their degradation and byproduct formation (IUPAC Technical Report)

Global climate change is a major concern as it leads to an increase in the average temperature of the earth’s atmosphere. The existence and persistence of some gaseous species in the atmosphere contribute to global warming. Experimental techniques are used to study the kinetics and degradation of global warming gases. However, quantum mechanical methods are also useful for the kinetic and radiative forcing study of global warming species and can precede experimental investigations. Research has also been targeted to develop more adapted procedures using ab initio and density functional theory (DFT) methods. This report provides a global perspective, in simplified manner, of the theoretical studies of the degradation of gas species in the atmosphere with an emphasis on the hydrogen abstraction kinetics of global warming gas species during their degradation and byproduct formation. En route, the results obtained from these studies are analysed and compared with experimental data where available. Our analyses indicate that the theoretical predictions are in agreement with experimental findings but the predicted parameters are dependent on the method being used. Theoretical methods are used to predict the thermodynamic parameters of reactions, and, with relevance to this report, the global warming potential (GWP) index can also be calculated. This report can be useful for future investigations involving global warming gaseous species while providing suggestions on how computations can fill in data gaps when experimental data are unavailable.

Empirical correction of nondynamical correlation energy for density functionals

To provide an efficient means to address nondynamical correlation, an empirical correction for single and double hybrid density functionals has been formulated. The correction utilizes the highest doubly occupied, lowest unoccupied, and singly occupied generalized Kohn-Sham orbitals and, for potential energy curves, a few additional strongly correlated orbitals. Utilizing proposed nondynamical correlation corrections, hybrid BLYP predicted potential energy curves that fit well to those obtained by ab initio multireference methods for the torsional rotation of ethylene and the automerization of cyclobutadiene. The empirical nondynamical correlation corrections, calculated at the transition structures, also reduce the overall errors of B2K-PLYP for the DBH24 and BH76 barrier height data sets. The proposed empirical correction can efficiently enhance the utility of both single and double hybrid density functionals for chemical situations with moderate to significant nondynamical correlation.