Pair-Density Functional Theory Based on the Spin-Projected Unrestricted Hartree-Fock Method

Spin-projected unrestricted Hartree-Fock (SUHF) theory is a valuable method that effectively addresses static correlation. To further enhance its accuracy, it is important to augment it with dynamic correlation. Based on SUHF theory, we propose a pair-density functional theory, namely, SU-PDFT, which formally follows the concept of multiconfiguration pair-density functional theory (MC-PDFT). SU-PDFT shows comparable accuracy to MC-PDFT but avoids the issue of the exponential scaling of the multiconfiguration (MC) approach. The hybrid version of SU-PDFT with one or two parameters is also developed, displaying further improved accuracy.