Q. Initial geometry for the SAC-CI optimization

 

A. SAC-CI geometry optimization uses Minimum Orbital Deformation (MOD) method (See details in the SAC-CI guide). Since the MOD method uses the MOs at the initial geometry as the reference orbitals, this could induce the initial-structure dependence in the optimized structure. Therefore, the initial geometry for the optimization is better to be close to the optimized structure. We provided MacroIteration option which allows repeating the optimization starting from the optimized geometry in the previous cycle. This option reduces such initial geometry dependence.