[ESPResSo-users] About the direction of the Gay-Berne (GB) interactions long axis

[Christian Häge]

Hi everyone,

I was looking at the GB interaction in the user ESPResSo User's Guide and couldn't help but notice that it's not written in which direction the long axis of the GB interaction is defined. Maybe it is written somewhere else and I just could not find it.
From what I've seen in the source code (especially the calculation of the quatu[3] array), I would say that the long axis of the GB-interaction is defined in direction (0,0,1) of the particles rotating body frame.

Question: Is it correct that the long axis of the GB-interaction is defined in direction (0,0,1) of the particles rotating body frame?


Also I noticed two other things: in the user manual (for version 3.3.1) on page 54 in equation 5.19 is a small error.
The first part of the equation written there is:

epsilon_0 * ( 1 - chi^2 * ( u_i * u_j ) )^(- nu/2)

according to
J. G. Gay, J. Chem. Phys. 1981, 74, 3316.
it should be:

epsilon_0 * ( 1 - chi^2 * ( u_i * u_j )^2 )^(- nu/2)

the change is marked in bold letters. I didn't know how to format an equation in an email. Sorry for the inconvenience.
However: The equation is correct in the source code (src/core/gb.hpp).

About the source code:
I think the following lines (lines 185 and 186) are incorrect in file src/core/gb.hpp:
The lines are:

    X = 1/(dist - Sigma + ia_params->GB_sig);
    Xcut = 1/(ia_params->GB_cut - Sigma + ia_params->GB_sig);

According to the User manual page 61 formula 5.17 they should be:

    X = ia_params->GB_sig / (dist - Sigma + ia_params->GB_sig);
    Xcut = ia_params->GB_sig / (ia_params->GB_cut - Sigma + ia_params->GB_sig);

This error will only have an effect if sigma_0 is not equal to 1.0.

Kind Regards & have a nice weekend,
Christian