Communication: Extension of a universal explicit electron correlation correction to general complete active spaces

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Multi-reference perturbation theories (MRPT) are essential for describing systems with strong electron correlation.
• Explicit correlation methods, like F12 corrections, aim to recover electron correlation effects missing in standard basis sets.
• Previous F12 corrections were limited in their applicability to specific multi-reference frameworks.

Purpose of the Study:

• To extend the universal explicit electron correlation (F12) correction to general complete active spaces.
• To apply this extended F12 correction to Mukherjee's multi-reference second-order perturbation theory (Mk-MRPT2).
• To assess the method's ability to reduce basis set incompleteness error.

Main Methods:

• Extension of the F12 correction formalism.
• Application to arbitrary complete active space self-consistent field (CASSCF) orbitals.
• Implementation within the Mk-MRPT2 framework.

Main Results:

• Successful extension of the F12 correction to general complete active spaces.
• Demonstrated applicability to Mk-MRPT2.
• Pilot examples showed a reduction in basis set incompleteness error by approximately two cardinal numbers.

Conclusions:

• The developed F12 correction is a versatile tool for improving the accuracy of MRPT calculations.
• This advancement significantly reduces the basis set incompleteness error in quantum chemical computations.
• The method offers a more reliable description of strongly correlated electronic systems.