# How to convert dihedral parameters from Tinker to GROMACS parameters (OPLS-AA)?

Hi,

I am trying to write an itp file for the compound 1,2-diethoxyethance (CCOCCOCC) (attached pdb to this post).

I used tinker analyze to obtain the parameter file, f2.out, which I have attached to this post. In order to check if my procedure was right, I downloaded an itp file from LigParGen (UNL_6B2B1D.itp).

Every parameter in my topology file (topol.itp) matched with the parameters from LigParGen, except the dihedrals with hydrogen.

The formulae I used to go from the Torsional Angle Parameters in tinker were the ones given in the GROMACS page: https://manual.gromacs.org/documentation/2020-beta3/reference-manual/functions/bonded-interactions.html, equation block 33.

If I look at the torsional angle parameters for torsion #11 in f2.out:

7 2 6 9 -0.521 0/1 -2.018 180/2 1.996 0/3

So, F1 = -0.521, F2 = -2.018, F3 = 1.996, F4 = 0

I use the GROMACS formula:

c0 = (F2+(F1+F3)/2)*4.184 = -5.358

c1 = 0.5*(-F1+3F3)*4.184 = 13.617

c2 = (-F2 + 4*F4)*4.184 = 8.443

c3 = -2F3*4.184 = -16.7025

c4 = -4F4 = 0

c5 = 0

However, I look at the UNL file, I see that:

7 2 6 9 3 1.590 4.770 0.000 -6.360 -0.000 0.000

Why is this the case? Where am I going wrong? Every other torsion matches the formula given in block 33 of the gromacs manual page.

I would appreciate any advice you have.
Relevant part of f2.out:

``````Torsional Angle Parameters :

Atom Numbers           Amplitude, Phase and Periodicity

1        3     1     2     6       0.468   0/3
2        3     1     2     7       0.300   0/3
3        3     1     2     8       0.300   0/3
4        4     1     2     6       0.468   0/3
5        4     1     2     7       0.300   0/3
6        4     1     2     8       0.300   0/3
7        5     1     2     6       0.468   0/3
8        5     1     2     7       0.300   0/3
9        5     1     2     8       0.300   0/3
10        1     2     6     9       0.650   0/1  -0.250 180/2   0.670   0/3
11        7     2     6     9      -0.521   0/1  -2.018 180/2   1.996   0/3
12        8     2     6     9      -0.521   0/1  -2.018 180/2   1.996   0/3
13        2     6     9    10       0.650   0/1  -0.250 180/2   0.670   0/3
14        2     6     9    11      -0.521   0/1  -2.018 180/2   1.996   0/3
15        2     6     9    12      -0.521   0/1  -2.018 180/2   1.996   0/3
16        6     9    10    13      -0.550   0/1
17        6     9    10    14       0.468   0/3
18        6     9    10    15       0.468   0/3
19       11     9    10    13       0.468   0/3
20       11     9    10    14       0.300   0/3
21       11     9    10    15       0.300   0/3
22       12     9    10    13       0.468   0/3
23       12     9    10    14       0.300   0/3
24       12     9    10    15       0.300   0/3
25        9    10    13    16       0.650   0/1  -0.250 180/2   0.670   0/3
26       14    10    13    16      -0.521   0/1  -2.018 180/2   1.996   0/3
27       15    10    13    16      -0.521   0/1  -2.018 180/2   1.996   0/3
28       10    13    16    17       0.650   0/1  -0.250 180/2   0.670   0/3
29       10    13    16    18      -0.521   0/1  -2.018 180/2   1.996   0/3
30       10    13    16    19      -0.521   0/1  -2.018 180/2   1.996   0/3
31       13    16    17    20       0.468   0/3
32       13    16    17    21       0.468   0/3
33       13    16    17    22       0.468   0/3
34       18    16    17    20       0.300   0/3
35       18    16    17    21       0.300   0/3
36       18    16    17    22       0.300   0/3
37       19    16    17    20       0.300   0/3
38       19    16    17    21       0.300   0/3
39       19    16    17    22       0.300   0/3
``````

Relevant part of UNL_6B2B1D.itp:

``````[ dihedrals ]
; PROPER DIHEDRAL ANGLES
;  ai    aj    ak    al funct            c0            c1            c2            c3            c4            c5
10    9    6    2        3       1.715   2.845   1.046  -5.607  -0.000   0.000
17   16   13   10        3       1.715   2.845   1.046  -5.607  -0.000   0.000
9    6    2    1        3       1.715   2.845   1.046  -5.607  -0.000   0.000
16   13   10    9        3       1.715   2.845   1.046  -5.607  -0.000   0.000
9    6    2    8        3       1.590   4.770   0.000  -6.360  -0.000   0.000
16   13   10   15        3       1.590   4.770   0.000  -6.360  -0.000   0.000
9    6    2    7        3       1.590   4.770   0.000  -6.360  -0.000   0.000
16   13   10   14        3       1.590   4.770   0.000  -6.360  -0.000   0.000
8    2    1    4        3       0.628   1.883   0.000  -2.510  -0.000   0.000
15   10    9   12        3       0.628   1.883   0.000  -2.510  -0.000   0.000
7    2    1    3        3       0.628   1.883   0.000  -2.510  -0.000   0.000
7    2    1    4        3       0.628   1.883   0.000  -2.510  -0.000   0.000
20   17   16   19        3       0.628   1.883   0.000  -2.510  -0.000   0.000
7    2    1    5        3       0.628   1.883   0.000  -2.510  -0.000   0.000
15   10    9   11        3       0.628   1.883   0.000  -2.510  -0.000   0.000
14   10    9   12        3       0.628   1.883   0.000  -2.510  -0.000   0.000
8    2    1    3        3       0.628   1.883   0.000  -2.510  -0.000   0.000
8    2    1    5        3       0.628   1.883   0.000  -2.510  -0.000   0.000
21   17   16   19        3       0.628   1.883   0.000  -2.510  -0.000   0.000
22   17   16   18        3       0.628   1.883   0.000  -2.510  -0.000   0.000
21   17   16   18        3       0.628   1.883   0.000  -2.510  -0.000   0.000
14   10    9   11        3       0.628   1.883   0.000  -2.510  -0.000   0.000
20   17   16   18        3       0.628   1.883   0.000  -2.510  -0.000   0.000
22   17   16   19        3       0.628   1.883   0.000  -2.510  -0.000   0.000
21   17   16   13        3       0.979   2.937   0.000  -3.916  -0.000   0.000
15   10    9    6        3       0.979   2.937   0.000  -3.916  -0.000   0.000
22   17   16   13        3       0.979   2.937   0.000  -3.916  -0.000   0.000
20   17   16   13        3       0.979   2.937   0.000  -3.916  -0.000   0.000
14   10    9    6        3       0.979   2.937   0.000  -3.916  -0.000   0.000
18   16   13   10        3       1.590   4.770   0.000  -6.360  -0.000   0.000
19   16   13   10        3       1.590   4.770   0.000  -6.360  -0.000   0.000
12    9    6    2        3       1.590   4.770   0.000  -6.360  -0.000   0.000
11    9    6    2        3       1.590   4.770   0.000  -6.360  -0.000   0.000
6    2    1    5        3       0.979   2.937   0.000  -3.916  -0.000   0.000
13   10    9   12        3       0.979   2.937   0.000  -3.916  -0.000   0.000
6    2    1    4        3       0.979   2.937   0.000  -3.916  -0.000   0.000
6    2    1    3        3       0.979   2.937   0.000  -3.916  -0.000   0.000
13   10    9   11        3       0.979   2.937   0.000  -3.916  -0.000   0.000
13   10    9    6        3      -1.151   1.151   0.000  -0.000  -0.000   0.000
``````