Can a DynMat calculation for an isolated molecule be done?Can a DynMat calculation for an isolated molecule be done?

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huuanh2811
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Can a DynMat calculation for an isolated molecule be done?Can a DynMat calculation for an isolated molecule be done?

Post by huuanh2811 » Fri Jul 05, 2019 4:11 am

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
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Re: Can a DynMat calculation for an isolated molecule be done?Can a DynMat calculation for an isolated molecule be done?

Post by graeme » Fri Jul 05, 2019 1:57 pm

That is exactly right - but make sure to set NSW to 25. It may still work with a larger value, but I'm not sure.

When you calculate the zero point energy, remember to remove the 6 near-zero modes, corresponding to translation and rotation.

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