Spin-symmetry adaptation to the Monte Carlo correction configuration interaction wave functions

Area of Science:

• Quantum Chemistry
• Computational Physics
• Electronic Structure Theory

Background:

• The Monte Carlo Correction Configuration Interaction (MC3I) method uses selected Slater determinants (SDs) to approximate wave functions.
• Spin-symmetry is often broken in MC3I wave functions due to the stochastic selection of SDs, particularly in systems with strong electron correlation.
• This symmetry breaking complicates the accurate assignment of electronic states, especially for excited states and complex molecular systems.

Purpose of the Study:

• To develop and implement a novel method for adapting spin-symmetry to MC3I wave functions.
• To restore the broken spin-symmetry in MC3I calculations, particularly for systems exhibiting strong electron correlation.
• To improve the reliability and applicability of the MC3I method for accurate electronic state determination.

Main Methods:

• Iterative application of the S^2 operator to the set of Slater determinants (SDs) within the MC3I wave function.
• Ensuring the set of SDs becomes closed under the S^2 operator, thereby restoring spin-symmetry.
• Diagonalization of the Hamiltonian matrix spanned by the spin-symmetry adapted set of SDs to obtain accurate wave functions.

Main Results:

• The spin-symmetry adaptation method successfully restored correct total spin angular momentum (S) values for challenging systems.
• Calculations for excited states of C2 and the [Fe2S2(SCH3)4]3- complex showed accurate S values after adaptation.
• The adapted MC3I results showed excellent agreement with established methods like Full Configuration Interaction (FCI) and Density Matrix Renormalization Group (DMRG).

Conclusions:

• The proposed spin-symmetry adaptation technique effectively resolves the issue of broken spin-symmetry in MC3I calculations.
• This method significantly enhances the MC3I approach, making it suitable for studying systems with strong electron correlation.
• The adapted MC3I method provides reliable electronic state assignments comparable to high-level quantum chemistry techniques.