Analytic gradient for the multireference Brillouin-Wigner coupled cluster method and for the state-universal...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Multireference coupled cluster (CC) methods are essential for accurately describing electronic structures of molecules with complex electronic correlations.
• Calculating molecular properties, such as gradients for geometry optimization, is computationally demanding for these methods.

Purpose of the Study:

• To develop and implement analytic gradient theory for multireference Brillouin-Wigner coupled cluster (BWCC) and state-universal coupled cluster methods.
• To provide a computationally efficient and accurate tool for molecular geometry optimizations.

Main Methods:

• Derivation of analytic gradients for three variants of BWCC: uncorrected, iteratively corrected for size-extensivity, and rigorously size-extensive state-universal CC.
• Pilot implementation utilizing full-configuration interaction expansions, limited to single and double excitations.
• Verification of analytic gradients against numerical calculations.

Main Results:

• Successful derivation and pilot implementation of analytic gradients for multireference CC methods.
• Demonstrated efficiency of the BWCC gradient implementation due to the absence of terms mixing different reference configurations.
• Validation of the analytic gradients using the CH2 molecule.

Conclusions:

• The developed analytic gradient theory and its implementation offer a significant advancement for multireference coupled cluster calculations.
• The methods are suitable for performing accurate geometry optimizations, as evidenced by calculations on CH2 and SiH2 molecules.