I am trying to find the binding of Na+ with some ligand using G09. In the solution phase calculation Na+ is binding as expected. However in the gas phase calculation (for solid sample) two bulky groups are rotating along a single bond which makes the binding of Na+ unfavorable. To tackle the problem I froze the dihedral angle. After the structure is optimized I had put an anion (BPh4) at a far away. The optimization of such a big structure is not possible (with PC) and crystal structure of the system is not available. But I am getting some negative frequency with such a structure and few modes are not matching with experiment (solid phase). Can anybody suggest how to solve the problem?