Multiconfiguration Pair-Density Functional Theory for Transition Metal Silicide Bond Dissociation Energies, Bond Lengths, and State Orderings.

Meagan S. Oakley,Donald G. Truhlar,Laura Gagliardi

doi:10.3390/molecules26102881

Abstract

Transition metal silicides are promising materials for improved electronic devices, and this motivates achieving a better understanding of transition metal bonds to silicon. Here we model the ground and excited state bond dissociations of VSi, NbSi, and TaSi using a complete active space (CAS) wave function and a separated-pair (SP) wave function combined with two post-self-consistent field techniques: complete active space with perturbation theory at second order and multiconfiguration pair-density functional theory. The SP approximation is a multiconfiguration self-consistent field method with a selection of configurations based on generalized valence bond theory without the perfect pairing approximation. For both CAS and SP, the active-space composition corresponds to the nominal correlated-participating-orbital scheme. The ground state and low-lying excited states are explored to predict the state ordering for each molecule, and potential energy curves are calculated for the ground state to compare to experiment. The experimental bond dissociation energies of the three diatomic molecules are predicted with eight on-top pair-density functionals with a typical error of 0.2 eV for a CAS wave function and a typical error of 0.3 eV for the SP approximation. We also provide a survey of the accuracy achieved by the SP and extended separated-pair approximations for a broader set of 25 transition metal–ligand bond dissociation energies.

Highlights

Transition metal silicides are useful in silicon-based devices due to their high thermal stability, low electric resistivity, and low density [1,2]
We study an inexpensive alternative to account for electron correlation energy, namely multiconfiguration pair-density functional theory (MC-PDFT) [25,26], which is here applied both with a CASSCF reference function (CAS-PDFT) and with an SP reference function (SP-PDFT) It has been previously shown that the MC-PDFT method has comparable accuracy to that of CASPT2 for dissociation energies, while requiring less computational resources, and it does not suffer from intruder state problems that often plague CASPT2 [15,26,27,28,29]
We report calculations of the ground state state symmetry, bond dissociation energy, equilibrium internuclear distance, and zero point energy for three transition metal silicides

Summary

Introduction

Transition metal silicides are useful in silicon-based devices due to their high thermal stability, low electric resistivity, and low density [1,2]. To obtain more accurate BDEs, the Morse group has utilized a precise predissociation-based resonant two-photon ionization method [6,7,8,9] that has an accuracy of approximately 0.004 eV This substantial increase in experimental accuracy produces benchmark values against which quantum chemistry methods can be tested, and the accuracy attainable can be a significant indicator of whether the bonding is modelled adequately. This kind of test of theory is interesting because prediction of the energetics of open-shell transition metal compounds is complicated by near-degeneracy effects that produce a plethora of low-lying electronic states [10,11] that can best be modelled with multiconfiguration quantum mechanical methods

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Conclusion