[gmx-users] force filed parameters for phosphoserine with gromaces

[YanhuaOuyang]

Dear Gromacs users,

I am going to run a MD of a 20-residue protein which is phosphorylated on the Serine and Threonine residues with AMBER ff99SB-ILDN force field using  Gromacs5.0. When I run gmx pdb2gmx and choose AMBER ff99SB-ILDN force field, it appears:
fatal error: residues SEP not found in residue topology database.

I search some papers, and they say that we need to add add force fields parameters for the phosphorylated amino acids to the force filed parameter files. But I do not know how to add force fields parameters for the phosphorylated amino acids. Could anyone knows how to construct a model for the phosphorylated protein?

Best regards,
Ouyang.