Diffusion quantum Monte Carlo calculations with a recent generation of effective core potentials for ionization potentials and electron affinities

Pseudopotentials are an essential ingredient in diffusion quantum Monte Carlo (DMC) calculations to increase efficiency substantially. A new generation of effective core potentials (ccECP) has been recently developed for DMC calculations. In this paper, performance of DMC using ccECP potentials on total energies, ionization potentials (IPs) and electron affinities (EAs) of some second- and third-row atoms and molecules is investigated systematically with different types of trial wave functions. DMC results are compared with those of high-level coupled-cluster methods extrapolated to complete basis set limit (CC-CBS). Error of ccECP potentials on IPs and EAs is also evaluated through a comparison with those from all-electron calculations. Our results show that mean errors in DMC energies with the ccECP potentials are smaller than those with the pseudopotentials developed by Burkatzki, Filippi, and Dolg (BFD), when the same type of trial wave functions is adopted. Mean absolute deviations (MADs) on IPs of DMC compared with those of CC-CBS are about 1.6 kcal/mol with single-determinant-Jastrow trial wave functions, and 1 kcal/mol with multideterminant-Jastrow trial wave functions using either ccECP or BFD potentials. MADs on EAs with DMC using the ccECP potentials are about 1 kcal/mol and slightly larger than those with the BFD potentials. Our results show that ccECP potentials are able to provide reliable IPs and EAs in DMC calculations. Accuracy of IPs and EAs from DMC calculations using ccECP potentials is similar to that with the BFD potentials, although mean error in total DMC energies with ccECP potentials is smaller. Furthermore, error of ccECP potentials in DMC calculations on IPs and EAs is smaller than that of BFD potentials compared with all-electron results.

Figure 1

Mean errors in DMC total energies for cations, anions as well as neutral species with different types of trial wave functions.

Figure 2

Percentage of the correlation energy (%CE) retrieved for each system with DMC using different types of trial wave functions.

Figure 3

Errors in DMC/CASSCF energies using the T-move scheme and locality approximation.

Figure 4

DMC energies of ${\mathrm{O}}_2^{+}$ with different types of trial wave functions using T-move and locality approximation schemes at different time steps.

Figure 5

Mean errors in DMC energies using ccECP and BFD potentials with different trial wave functions and nonlocal-treating strategies.

Figure 6

Errors in DMC/UB3LYP energies using ccECP and BFD potentials with the T-move scheme.