I am looking for rotational barriers (and shapes of energy vs dihedral angle) for a series of organic molecules having single, double and triple C-C middle bond (ethane, ethene, ethyne, propane, propene, propyne, butane, butene, butyne and their radicals). I do it in two steps:
1. rotate the molecule to get certain dihedral angle
2. perform energy minimisation with fixed dihedral angle
I am doing it in VASP, so it is a little bit tricky as this program does not allow for fixing only the certain dihedral angle. Only whole atoms can be fixed, but this fix also bond distances, which I don't want to be fixed.
So it goes quite slow, so I thought that I can try Gaussian for this purpose. I have tried it for the last 3 days, so I am even not a beginner - just found somewhere some input file and tried to use it:
%nproc=1
%mem=1500MB
#B3LYP/3-21g Opt=ModRedundant
test job
0 1
C1 6.5830579 8.8652487 8.3335590
C2 7.9166780 8.5190792 8.9943390
C3 9.0833178 8.4809189 8.0056591
C4 8.6628351 8.7972202 6.5710335
H5 5.7602983 8.8865595 9.0637493
H6 6.3232479 8.1277695 7.5573192
H7 6.6268883 9.8538189 7.8490591
H8 7.8389683 7.5389791 9.4991589
H9 8.1405783 9.2540388 9.7888288
H10 9.8579645 9.1978540 8.3334131
H11 9.5563774 7.4828029 8.0437136
H12 8.2156544 9.8018513 6.5026355
H13 7.9119382 8.0757952 6.2109599
H14 9.5178509 8.7636509 5.8793764
* 2 3 * F
I wanted to fixed atom 1-2-3-4 dihedral angle, but I probably fixed all dihedral angles contains atoms 2 and 3. But anyway I am sure that I missed a lot of stuff. However when I did such calculation for butane 0 deg ang 180 deg (according to the central C-C bond) I got really similar energy barriers: 5.544 kcal / mol for VASP and 5.529 kcal / mol for Gaussian. Results are similar but I spent ~2 minutes for waiting for Gaussian results and much more (days or even weeks, due to problems with clusters on which I run my VASP calculations) for VASP.
So the question is if this Gaussian input file is acceptable or if not how to modify it to allow for including vdW interaction, spin, magnetic moment change (breaks of triple bond during rotation of the molecule)?
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