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Symmetry Problem

Dong Xu
dongxu@sciences.sdsu.edu

I have an MP2/aug-cc-pVDZ job with the molecule being in C2h symmetry and it ran into this error:

MP2 TRANSFORMATION CANNOT ASSIGN A DEFINITE ORBITAL SYMMETRY TO MO  278
JOB ABORTS.  YOU MUST EITHER
 A) PROVIDE A SYMMETRY ADAPTED MO SET BY GUESS=MOREAD
 B) ENTER NOSYM=1 TO USE THESE SYMMETRY BROKEN MO-S.

The problem is there are two 90 degree dihedral angles in this molecule (C2h symmetry) that I want to freeze throughout the calculation. If I assign NOSYM=1 to diable C2h symmstry, the optimization will turn the dihedral angles to 0 degree and find the global minimum. I didn't have this problem in my previous MP2/cc-PVDZ or MP/cc-pVTZ jobs.

So, what's the best strategy here to finish this calculation and keep the geometry staying at this saddle point (two 90 degree dihedral angles)?

Should I use RUNTYP=SADPOINT or IFREEZ/IFCART to freeze the coordinates, and how? I have COORD=CART, a full Cartisian coordinate geometry input.

Thanks in advance and Happy holidays!

-DX

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