I don't have access to Loukas Peristeras's email address. Steve can

you forward this question to him?

Problem:

The current version of dihedral_fourier.cpp, each cosine term can be

shifted, but it must be shifted by an integer number of degrees. This

shift is the "di" parameter mentioned in the documentation for that

dihedral_style at:

http://lammps.sandia.gov/doc/dihedral_fourier.html

A shift of 62.5 degrees is not allowed.

I can't think of a reason for this. In the code, Loukas said he did

this for "backwards compatibility". But I I don't understand why we

need backwards compatibility for a new dihedral style. (Was this part

of the code accidentally left over from dihedral_charmm.cpp? If so,

then great, we can fix it easily. See below.)

I am grateful, excited. But I am also disappointed. This is a

significant problem, especially for large Fourier series. Perhaps

this limitation was not so serious for dihedral_style charmm. However

a shift of half-a-degree is serious for longer fourier series. And

people who want long Fourier series are the target audience.

We actually use Fourier-series dihedral potentials, where a shift of

half a degree leads to qualitatively differences in the behavior of

our coarse-grained protein model. (Bellesia, G.; Jewett, A. I.; Shea,

J.-E. Protein Sci. 2010, 19, 141-154)

Can we please remove this restriction for this new pair style? I've

attached a slightly modified version of "dihedral_fourier.cpp" which

allows for non-integer angle shifts (based on the Oct28_2012 version

of LAMMPS). I only made modifications on lines 316 and 337 of that

file.

Dear Loukas

Please let us know if you are open to the idea of non-integer

shifts. Our group uses especially non-standard complicated

potentials. We would be grateful if you are willing to change this.

I don't mean to sound so crestfallen and unappreciative. I think

it's wonderful that you wrote this new dihedral style, and I intend to

use it (and get other people using it).

Thanks!

Andrew

dihedral_fourier.cpp (11.1 KB)