Molecular applications of analytical gradient approach for the improved virtual orbital-complete active space...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate determination of molecular geometries and vibrational frequencies is crucial for understanding chemical properties and reactions.
• Existing methods like Complete Active Space Self-Consistent Field (CASSCF) can suffer from convergence issues.

Purpose of the Study:

• To extend the improved virtual orbital-complete active space configuration interaction (IVO-CASCI) method for geometry and vibrational frequency calculations.
• To assess the applicability of IVO-CASCI for ground, excited, and open-shell electronic states.
• To compare IVO-CASCI performance against established theoretical methods.

Main Methods:

• Utilized an analytical total energy gradient scheme with first and second-order derivatives.
• Applied the IVO-CASCI method to calculate ground state geometries of benzene, biphenyl, and alanine dipeptide.
• Performed IVO-CASCI geometry optimization for excited singlet and triplet states of benzene.

Main Results:

• IVO-CASCI successfully determined ground state geometries for all studied molecules.
• Optimization of benzene's triplet state yielded a stable minimum after identifying a saddle point.
• IVO-CASCI results showed comparable or superior performance to Hartree-Fock, MP2, CASSCF, and DFT methods.
• Computed vibrational frequencies for benzene and biphenyl agreed well with experimental data and other accurate theoretical estimates.

Conclusions:

• The IVO-CASCI method is effective for calculating geometries and vibrational frequencies of various electronic states.
• IVO-CASCI demonstrates comparable accuracy to CASSCF but with improved computational efficiency and convergence properties.
• The method shows significant potential as a replacement for CASSCF in computational chemistry studies.