Hi,
in gaussian FTIR log file, all force constant units are mdyne/A. It doesn't matter for gaussian the vibration belongs to bond or angle it uses the same unit. But Amber gaff file uses kcal/mol/radian^2 (http://alma.karlov.mff.cuni.cz/bio/99_Studenti/00_Dalsi/ParamFit/2013_ParamFit_AmberTools13.pdf page:60) and I couldn't convert gaussian force constants to gaff for angles. How can I overcome this problem.
Gaussian gives stretching and bending force constants in the same units, mdynes/Å. You can convert 1 mdynes·Å-1 = 100 N·m-1 = 100 J·m-2. The conversion into kcal·mol-1·Å-2 is straightforward, but the following into kcal·mol-1·rad-2 may not be obvious. Particularly, from Å-2 to rad-2.
Remember that a radian is given by the radius of the circle in which it is defined. You can consider the Bohr radius for such case, so that 1 rad = 0.52918 Å. Then you have that 1 kcal·mol-1·Å-2 = 0.28003 kcal·mol-1·rad-2.
In page 503 of the book Introduction to Infrared and Raman Spectroscopy by N. Colthup (Elsevier, 2nd Ed., 2012), it is stated that "sometimes bending force constants are expressed in the same units as stretching force constants, mdynes/Å. These are obtained [...] by dividing pure bending force constants by the square of a scaling factor in Å [...]. It can be a bondlength included in the angle". What I did above is to multiply by the square of a scaling factor, which is the opposed transformation what you required, with that scaling factor being the Bohr radius, which I think is a more general approach (since it is the atomic unit of length).
Gaussian gives stretching and bending force constants in the same units, mdynes/Å. You can convert 1 mdynes·Å-1 = 100 N·m-1 = 100 J·m-2. The conversion into kcal·mol-1·Å-2 is straightforward, but the following into kcal·mol-1·rad-2 may not be obvious. Particularly, from Å-2 to rad-2.
Remember that a radian is given by the radius of the circle in which it is defined. You can consider the Bohr radius for such case, so that 1 rad = 0.52918 Å. Then you have that 1 kcal·mol-1·Å-2 = 0.28003 kcal·mol-1·rad-2.
In page 503 of the book Introduction to Infrared and Raman Spectroscopy by N. Colthup (Elsevier, 2nd Ed., 2012), it is stated that "sometimes bending force constants are expressed in the same units as stretching force constants, mdynes/Å. These are obtained [...] by dividing pure bending force constants by the square of a scaling factor in Å [...]. It can be a bondlength included in the angle". What I did above is to multiply by the square of a scaling factor, which is the opposed transformation what you required, with that scaling factor being the Bohr radius, which I think is a more general approach (since it is the atomic unit of length).
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