Improved virtual orbitals in state specific multireference perturbation theory for prototypes of quasidegenerate...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Quasidegeneracy presents significant challenges in accurately calculating molecular properties.
• Traditional methods like Complete Active Space Self-Consistent Field (CASSCF) often face convergence difficulties with larger active spaces.

Purpose of the Study:

• To investigate molecular systems where quasidegeneracy effects are significant.
• To evaluate the performance of the improved virtual orbital complete active space configuration interaction state-specific multireference perturbation theory (IVO-SSMRPT) method.

Main Methods:

• Application of the IVO-SSMRPT method to model systems like BeH2, BeCH2, MgCH2, Si2H4, H2CO dissociation, and 1,3-butadiene intramolecular reactions.
• Utilizing IVO-CASCI as a reference function, which avoids iterative convergence issues beyond the initial SCF calculation.
• Employing SSMRPT as a second-order perturbative approximation to capture remaining correlation energy.

Main Results:

• The IVO-SSMRPT method successfully avoids intruder states in a size-extensive manner.
• The method allows for revision of the wavefunction content within the model space.
• Accurate predictions of energy surfaces, geometrical parameters, and spectroscopic properties were achieved.

Conclusions:

• The IVO-SSMRPT method provides predictive accuracy for systems with quasidegeneracy.
• Results show good agreement with high-level theoretical calculations and experimental findings.
• This method offers a robust alternative to CASSCF for challenging electronic structure problems.