SSFEP Using the SilcsBio GUI

Please see SSFEP Simulations Using the GUI in the Graphical User Interface (GUI) Quickstart for instructions on running SSFEP from the SilcsBio GUI.

SSFEP Using the CLI

The following usage details are provided for completeness. We strongly recommend using the SilcsBio GUI to set up, run, and analyze SSFEP calculations.

To perform the SSFEP precomputation simulations, protein coordinates in PDB file format and parent ligand coordinates in Mol2 file format are required. The protein should have termini properly capped, missing loops built or the ends of the missing loops capped, standard atom and residue names, and sequential atom and residue numbering. Using these two files, run the following:

${SILCSBIODIR}/ssfep/1_setup_ssfep prot=<Protein PDB> lig=<Ligand Mol2/SDF>


The setup program internally use the GROMACS utility pdb2gmx, which may have problems processing the protein PDB file. The most common pdb2gmx issue involves mismatches between the expected residue name/atom names in the input PDB and those defined in the CHARMM force-field.

To fix this problem: Run the pdb2gmx command manually from within the 1_setup directory for a detailed error message. Please contact for additional assistance.

Following completion of the setup, run 10 MD jobs:

${SILCSBIODIR}/ssfep/2_run_md_ssfep prot=<Protein PDB> lig=<Ligand Mol2/SDF>

This command will submit 10 jobs to the pre-defined queue: 5 for the ligand in water and 5 for the ligand complexed with protein.

Once the precomputation simulations are completed, the 2_run_md/1_lig/[1-5] and 2_run_md/2_prot_lig/[1-5] directories will contain *.1-10.whole.trr trajectory files. If these files are not generated, then your simulations are either still running or have stopped due to a problem. Look into the log files within these directories for an explanation of the failure.

Ligand Modifications

Follow the instructions in Chemical Group Transformations to create modifications to your parent ligand.

Evaluating Binding Affinity Changes

Once modifications.txt has been prepared and the MD simulations involving the parent ligand are completed, run the following script to set up a \(\Delta \Delta G\) calculation.

${SILCSBIODIR}/ssfep/3a_setup_modifications prot=<Protein PDB> lig=<Ligand Mol2/SDF File> mod=modifications.txt

This will submit 10 jobs to evaluate all snapshots from the completed MD simulations of the parent ligand in order to calculate the change in free energy for every modification specified in your modifications.txt. Structures of these modifications in Mol2 format are output as 3_analysis_<modified ligand name entry in modifications.txt>/mod_files/*.mol2.

After these jobs complete, you may obtain \(\Delta \Delta G\) for your full list of modifications using:

${SILCSBIODIR}/ssfep/3b_calc_ddG_ssfep mod=modifications.txt

Example output follows: