Toward Compact Selected Configuration Interaction Wave Functions with Quantum Monte Carlo─A Case Study of C2.

The 1Σg+ ground state of C2 is investigated using truncated CIPSI-Jastrow CSF wave functions with Hartree-Fock orbitals within the framework of variational and diffusion quantum Monte Carlo. The truncation is performed based on the absolute value of the CI coefficients, and the Jastrow, molecular orbitals, and CI parameters are either partially or fully reoptimized with respect to the variational energy. Excellent absolute as well as bond dissociation energies are obtained at DMC level with very compact, fully optimized wave functions. By studying the expansions in more detail, we observe a change in the CI picture when reoptimizing the antisymmetric part of the CIPSI-Jastrow wave functions. Furthermore, we demonstrate that a decrease in the VMC energy as well as an improvement of the nodal surface quality can be achieved─with the same expansion size─if the CSFs are selected in the presence of a Jastrow correlation function, laying the foundation for a Jastrow selected CI scheme with quantum Monte Carlo.

Determination of the semiexperimental equilibrium structure of 2-acetylthiophene in the presence of methyl internal rotation and substituent effects compared to thiophene.

The microwave spectra of thiophene and 2-acetylthiophene were recorded in the frequency range from 2 to 40 GHz using two molecular jet Fourier transform microwave spectrometers. For 2-acetylthiophene, two conformers with a syn and an anti orientation of the S1-C2 and C6O bonds (with respect to the C2-C6 bond) were identified, and the syn-conformer was more stable. The spectra of the 34S- and 13C-isotopologues of syn-2-acetylthiophene were also assigned, and the semiexperimental equilibrium structure could be determined. Compared to thiophene, at the substitution position, the S1-C2 and C2C3 bond lengths both increase by about 0.007 Å, and the bond angle S1-C2C3 decreases by 0.06°, noticeably larger than the experimental uncertainties. A-E torsional splittings were observed due to internal rotation of the methyl group hindered by a barrier height of 330.187(35) and 295.957(17) cm-1 for the syn-conformer and the anti-conformer, respectively. Geometry and internal rotation parameters are compared with those of related thiophene derivatives, as well as those of furan and 2-acetylthiophene to gain a better understanding of structure determination in the presence of methyl internal rotation.

Full Wave Function Optimization with Quantum Monte Carlo-A Study of the Dissociation Energies of ZnO, FeO, FeH, and CrS.

The dissociation energies of four transition-metal dimers are determined using diffusion Monte Carlo. The Jastrow, configuration interaction, and molecular orbital parameters of the wave function are both partially and fully optimized with respect to the variational energy. The pivotal role is thereby ascribable to the optimization of the molecular orbital parameters of a complete active space wave function in the presence of a Jastrow correlation function. Excellent results are obtained for ZnO, FeO, FeH, and CrS. In addition, potential energy curves are computed for the first three compounds at the multireference diffusion Monte Carlo (MR-DMC) level, from which spectroscopic constants such as the equilibrium bond distance, the harmonic frequency, and the anharmonicity are extracted. All of these quantities agree well with the experiment. Furthermore, it is shown for CrS that a restricted active space calculation can yield improved initial orbitals by including single and double excitations from the original active space into a set of virtual orbitals. We demonstrated in this study that the fixed-node error in DMC can be systematically reduced for multireference systems by orbital optimization in compact active spaces. Although DMC calculations with a large number of determinants are possible and very accurate, our results demonstrate that compact wave functions may be sufficient in order to obtain accurate nodal surfaces, which determine the accuracy of DMC, even in the case of transition-metal compounds.