[gmx-users] OPLS and RB parameters in GROMACS

[sulatha M. S]

Hi all,

Hi



I've converted the OPLS-AA torsional potential parameters for the

alkane C-C-C-C (CT-CT-CT-CT in the gromacs parameter file notation),

C-C-C-H (CT-CT-CT-HC), and H-C-C-H (HC-CT-CT-HC) torsions from the

OPLS format given in Jorgensen et al, JACS 118, 11225 (1996)

to the Ryckaert-Bellemans format given in ffoplsaabon.itp and found

that the calculated values are different.



A previous post to the gmx-users mailing list on March 27, 2008,

pointed out this issue for the H-C-C-H torsional potential but there
was no response to that.



Does anyone know if there is an error in the ffoplsaabon.itp file? Or

is there a newer set of OPLS-AA parameters?



For the OPLS-AA parameters (in kcal/mol), I used:



dihedral              V1          V2             V3

C-C-C-C            1.740      -0.157         0.279

C-C-C-H             0.0          0.0            0.366

H-C-C-H             0.0          0.0            0.318



from which I calculated the Ryckaert-Bellemans parameters (in kJ/mol) as:



dihedral              C0          C1             C2           C3

C-C-C-C            3.56686   -1.889076   0.65688       -2.33467

C-C-C-H             0.66526   1.99577     0.0           -2.661024

H-C-C-H             0.76567   -2.29702    0.0           -3.06269



the parameters in the ffoplsaabon.itp file are:



dihedral              C0          C1             C2           C3

C-C-C-C           2.9288      -1.4644      0.2092     -1.6736

C-C-C-H           0.6276       1.88280    0.0           -2.5104

H-C-C-H           0.6276       1.8828      0.0           -2.5104



Thankyou for any clarification.

Sulatha
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