For treating both relativistic effect and electron correlation, the spin-free exact two-component and spin-dependent first-order Douglas-Kroll-Hess (sf-X2C-so-DKH1) Hamiltonian and the state-interaction (SI) method are combined to calculate the spin-orbit coupling (SOC) on multi-configuration electron correlation theory. Here, SOC is evaluated via SI among the spin-free states from the complete active space self-consistent field (CASSCF) calculation, and the dynamic electron correlation could be reckoned via the high-level multi-reference electron correlation method. Work equations to evaluate SOC matrix elements over spin-adapted Gelfand states in the framework of the graphic unitary group approach (GUGA) are presented. Benchmark calculations have verified the validity of the present implementation. As a pilot application, the internally contracted MRCI (icMRCI) with the inclusion of SOC calculation produces the reasonable equilibrium bond length and the harmonic vibrational frequency of the ground state of AuO, as well as the transition energy of $X^2\Pi_{3/2} \leftarrow ^2\Pi_{1/2}$.

GridMol is a “one-stop” platform for molecular modeling, scientific computing and molecular visualization aided by High Performance Computing Environment. GridMol version 2.0 emphatically introduces two unique features, the first is fragment-based linear scaling quantum chemistry methods, such as molecular fractionation with conjugate caps and fragment molecular orbital methods; the second is visualization of computational processes, such as structural optimization and intrinsic reaction coordinate calculation. Compared with version 1.0, fragment-based linear scaling quantum chemistry methods implemented in GridMol version 2.0 can be used as a useful tool for performing quantum calculations for large molecular systems to explore the mechanisms involved in protein–ligand or targeted-drug interactions.