Diffusion Monte Carlo for Accurate Dissociation Energies of 3d Transition Metal Containing Molecules

Abstract

Transition metals and transition metal compounds are important to catalysis, photochemistry, and many superconducting systems. We study the performance of diffusion Monte Carlo (DMC) applied to transition metal containing dimers (TMCDs) using single-determinant Slater-Jastrow trial wavefunctions and investigate the possible influence of the locality and pseudopotential errors. We find that the locality approximation can introduce nonsystematic errors of up to several tens of kilocalories per mole in the absolute energy of Cu and CuH if Ar or Mg core pseudopotentials (PPs) are used for the 3d transition metal atoms. Even for energy differences such as binding energies, errors due to the locality approximation can be problematic if chemical accuracy is sought. The use of the Ne core PPs developed by Burkatzki et al. (J. Chem. Phys. 2008, 129, 164115), the use of linear energy minimization rather than unreweighted variance minimization for the optimization of the Jastrow function, and the use of large Jastrow parametrizations reduce the locality errors. In the second section of this article, we study the general performance of DMC for 3d TMCDs using a database of binding energies of 20 TMCDs, for which comparatively accurate experimental data is available. Comparing our DMC results to these data for our results that compare best with experiment, we find a mean unsigned error (MUE) of 4.5 kcal/mol. This compares well with the achievable accuracy in CCSDT(2)Q (MUE = 4.6 kcal/mol) and the best all-electron DFT results (MUE = 4.5 kcal/mol) for the same set of systems (Truhlar et al. J. Chem. Theory Comput. 2015, 11, 2036-2052). The mean errors in DMC depend less on the exchange-correlation functionals used to generate the trial wavefunction than the corresponding mean errors in the underlying DFT calculations. Furthermore, the QMC results obtained for each molecule individually vary less with the functionals used. These observations are relevant for systems such as molecules interacting with transition metal surfaces where the DFT functionals performing best for molecules (hybrids) do not yield improvements in DFT. Overall, the results presented in this article yield important guidelines for both the assessment of the achievable accuracy with DMC and the design of DMC calculations for systems including transition metal atoms.

Quantum Monte Carlo Calculations on a Benchmark Molecule–Metal Surface Reaction: H2 + Cu(111)

“…Optimizing with respect to energy has proven beneficial in minimizing locality errors in the DMC calculations. 51 Furthermore, using the same preoptimized parameters for both the transition-state geometry and the asymptotic geometry will ensure the best possible error cancellation of locality errors when taking energy differences. The influence of residual locality errors will be discussed later.…”

“…The PPs are treated using the T-move scheme 28,63 that has been proven to be beneficial in terms of the size of locality errors. 51 …”

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

“…Horváthová et al 16 presented energetics for transition metal organometallics employing QMC methods. Recently, Doblhoff-Dier et al 17 published dissociation energies of 3d transition metal compounds calculated with DMC.…”

“…The authors of that work also state a prominent multi-reference character for those compounds. ZnO and FeH were also examined by Doblhoff-Dier et al 17 using DMC with Kohn-Sham (KS) initial orbitals. The significant deviation to the experimental dissociation energies again suggests that a single-reference approach is not able to correctly describe these systems.…”

Charge Density and Redox Potential of LiNiO2 Using Ab Initio Diffusion Quantum Monte Carlo

“…Therefore, DFT based methods have been largely utilized in periodic systems. In this approach the nodal surface is often optimized by either varying the U interaction parameter or the exact exchange ratio in the hybrid-DFT functionals [41,45,63,64]. Second, Jastrow parameters (correlation functions) are added to the guiding wavefunction and then optimized to further capture manybody correlations in the system.…”