Accurate spectroscopic properties by diffusion quantum Monte Carlo calculations

The capability of Diffusion Quantum Monte Carlo (DMC) to produce high quality potential energy curve (PEC) was evaluated. H-2(+), HeH+ and LiH PECs were built by all-electron fixed-node DMC calculations. Trial wave functions were obtained from Hartree-Fock (HF) (H-2(+)), MCSCF and CI (HeH+ and LiH) calculations multiplied by Jastrow factor. The quality of these generated PECs was analyzed throughout equilibrium distance, dissociation energy, vibrational energies and rovibrational spectroscopic constants (omega(e), omega(e)x(e), omega(e)y(e), alpha(e), gamma(e) and B-e). The Discrete Variable Representation (DVR) and the Dunham approaches were used to determine the rovibrational spectroscopic constants. The PECs and the aforementioned properties were also obtained by the following methods: MCSCF/aug-cc-pV5Z (LiH), CCSD(T)/aug-cc-pV5Z (HeH+ and LiH) and HF (H-2(+) and HeH+) levels. The results of these DMC computations, specially the DMC-DVR procedure, are the most accurate among others DMC calculations available in the literature for these systems. They suggest that DMC can be used to achieve accurate PECs to produce spectroscopic properties with the same level of accuracy of theoretical benchmarks and experimental data of the literature. (C) 2020 Elsevier B.V. All rights reserved.