Extended separated-pair approximation for transition metal potential energy curves.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Materials Science

Background:

• Accurate theoretical prediction of bond dissociation energies and potential energy curves is crucial for transition-metal chemistry.
• Developing computationally efficient yet accurate methods remains a significant challenge in this field.

Purpose of the Study:

• To investigate the performance of multiconfiguration pair-density functional theory (MC-PDFT) for transition-metal diatomics.
• To evaluate MC-PDFT using wave functions from complete-active-space (CAS) and generalized active space (GAS) self-consistent-field (SCF) methods.
• To compare MC-PDFT with established methods like CAS second-order perturbation theory (CASPT2) and Kohn-Sham DFT (KS-DFT).

Main Methods:

• Employed six systematic active space selection schemes, including correlated participating orbitals (CPO) and novel separated-pair (SP) and extended separate pairs (ESP) approximations.
• Applied CAS-SCF and GAS-SCF frameworks to calculate wave functions for MC-PDFT.
• Tested methods on TiC, TiSi, and WCl diatomics, comparing predicted bond energies and potential curves with experimental data.

Main Results:

• MC-PDFT demonstrated superior performance compared to both CASPT2 and KS-DFT across the studied transition-metal diatomics.
• The SP and ESP approximations significantly reduced computational costs and smoothed potential energy curves, particularly ESP-PDFT.
• ESP-PDFT achieved accuracy comparable to the more computationally intensive CAS-PDFT.

Conclusions:

• MC-PDFT provides a robust and accurate computational approach for transition-metal chemistry.
• The ESP approximation offers a computationally efficient strategy for active space selection within MC-PDFT.
• This work highlights the potential of MC-PDFT with optimized active spaces for reliable theoretical predictions in transition-metal systems.