Can a DynMat calculation for an isolated molecule be done?

[jianmin]

Dear guys,

I need the zero point energy (ZPE) of a gas phase molecule like ethane. My quesitions are:
1. Can we get the vibration frequencies of an isolated molecule in a big box by a Dynamical Matrix calculation?
2. If we can, how do I prepare the DISPLACECAR file? Take ethane (8 atoms) as an example, if I want the displacement to be 0.001, should I just write 8 lines of 0.001, 0.001, 0.001 in the DISPLACECAR file?
3. How do I specify NSW? Is it equal to 3*8+1=25? If I specify NSW to be more than 25, for instance, 30, would I still get the right results?

Thanks a lot.

[graeme]

Yes, that is exactly right. Just make sure to remove the 6 lowest modes (rotation and translation) from the modes.dat file before calculating the zero point energy.

Well, and set NSW correctly; don't make it larger than it should be. It might still work, but the program might also hang.

[jianmin]

Prof. Graeme, I appreciate your reply. I will follow your suggestions.