“…A HLSP function is expressed as a linear combination of Slater determinants, which are constructed from strictly localized HAOs, while a spin‐free VB wave function is defined by using the projection operator of the symmetric group. [5, 15] In any case, the VB energy can be written as, In the VBSCF method, the structural coefficients C and the VB orbitals ϕ m are optimized simultaneously by minimizing the total energy, which is determined by solving following secular equation: Here, H VB and M VB are the Hamiltonian and overlap matrices between VB structures, defined as, The VBSCF structure weights can be evaluated by using Coulson‐Chirgwin formula,[25] as For a CAS, where all independent structures are involved in VBSCF calculation, the VBSCF(CAS) wave function is invariant under transformation of the active orbitals. Thus, nonorthogonal active VB orbitals and the inactive orbitals can be orthogonalized without loss of generality, and the procedure keeps the VBSCF wave function invariant.…”