Quick Start

This document provides the basic usage process of s_mmpbsa to help you quickly get started with this tool for binding free energy calculations.

Starting s_mmpbsa

After installation, you can start s_mmpbsa in the following ways:

# Run directly in the command line
s_mmpbsa

# Or specify the tpr file path directly
s_mmpbsa md.tpr

After starting, you will see the welcome message of s_mmpbsa and then enter the interactive interface.

Basic Workflow

s_mmpbsa’s basic workflow includes the following steps:

  1. Load input files (tpr, xtc and ndx files)

  2. Set trajectory parameters (select receptor and ligand groups)

  3. Set MM-PBSA parameters

  4. Execute calculation

  5. Analyze results

We will detail the operation method of each step below.

Loading Input Files

After starting s_mmpbsa, you first need to load the necessary input files:

# Input tpr file path
md.tpr

# Load xtc file (option 1)
1
md_centered.xtc  # If the trajectory has been processed with PBC, you can input it directly; otherwise press Enter to use the default md.xtc

# Load ndx file (option 2)
2
index.ndx  # Press Enter to use the default index.ndx

# Proceed to the next step (option 0)
0

Setting Trajectory Parameters

After loading input files, you need to set trajectory parameters, mainly selecting receptor and ligand groups:

# Select receptor group (option 1)
1
[Select the number of the receptor group, for example 1 represents Protein]

# Select ligand group (option 2)
2
[Select the number of the ligand group, for example 13 represents ligand]

# Set time interval (option 5), usually analyze once every 1ns
5
1  # Time interval, unit is ns

# Proceed to the next step (option 0)
0

Setting MM-PBSA Parameters

Next, set the relevant parameters for MM-PBSA calculation:

# Under normal circumstances, you can use the default parameters
# If you need to modify PB parameters, you can select option 8
# If you need to modify SA parameters, you can select option 9

# Proceed to the next step (option 0)
0

Executing Calculation

After setting up, start executing the calculation:

# Input system name (default is system)
[Press Enter to use the default name or input a custom name]

# Wait for calculation to complete
# A progress bar and current energy value will be displayed during calculation

Analyzing Results

After calculation is complete, you can analyze the results:

# Generate pdb file containing residue binding energy information (option -1)
-1
[Press Enter to use the default time point (average value) or input a specific time point]

# View result summary (option 1)
1

# Output energy change data over time (option 2)
2

# Output residue binding energy at a specific time point (option 3)
3
[Press Enter to use the default time point (average value) or input a specific time point]
1  # Select to output residues within 3Å range

# Output binding energy of ligand atoms (option 4)
4

# Exit program (option 0)
0

Using Analysis Mode

s_mmpbsa also provides a special analysis mode, which can re-analyze already calculated results:

# Start analysis mode
s_mmpbsa
a  # Input 'a' to enter analysis mode

# Input working directory path (default is current directory)
[Press Enter to use current directory or input the directory containing .sm result files]

# Input temperature (default is 298.15K)
[Press Enter to use default temperature or input custom temperature]

# Input system name (default is system)
[Press Enter to use default name or input the system name used during previous calculation]

# Subsequent analysis operations are the same as after normal calculation completion

Example: Calculating Protein-Ligand Binding Energy

The following is a complete example of calculating protein-ligand binding energy:

# Start s_mmpbsa and load files
s_mmpbsa
md.tpr
1
md_centered.xtc
2
index.ndx
0

# Set trajectory parameters
1
1  # Assume 1 is the Protein group
2
13  # Assume 13 is the ligand group
5
1
0

# Set MM-PBSA parameters (use default values)
0

# Execute calculation
protein_ligand  # System name

# Analyze results
-1
1
2
3
1
4
0

Usage Tips

  1. Trajectory preparation: Before calculation, it is recommended to use Gromacs’ trjconv tool to process the trajectory, including removing PBC, centering and fitting operations, to obtain better calculation results.

  2. Index file: Ensure that the index file contains correct receptor and ligand groups. If there is no ready-made index file, you can use Gromacs’ make_ndx tool to create one.

  3. Time interval: For long MD simulations, you can appropriately increase the time interval to reduce the calculation amount. Usually analyzing once every 1-2ns can obtain good statistical results.

  4. Parallel computing: Setting an appropriate nkernels value in settings.ini can utilize multi-core CPU to accelerate calculation.

  5. Result visualization: The generated pdb file can be opened with software such as PyMOL to view the distribution of residue binding energy (colored by B factor).

Frequently Asked Questions

### How to handle large systems?

For large systems, you can try the following optimization measures:

  • Increase the time interval to reduce the number of analyzed frames

  • Increase the nkernels value to utilize more CPU cores

  • Use smaller cutoff distance (by modifying the r_cutoff parameter)

### How to improve calculation accuracy?

Methods to improve calculation accuracy include:

  • Ensure good trajectory quality and correct PBC handling

  • Increase the number of sampling points, i.e., reduce the time interval

  • Adjust PB parameters, such as grid size, solvent dielectric constant, etc.

### How to interpret calculation results?

A larger negative value of binding free energy indicates stronger binding. Usually, the calculation results will give the following energy terms:

  • ΔG_bind: Total binding free energy

  • ΔH: Enthalpy change

  • TΔS: Entropy contribution

  • ΔE_vdw: Van der Waals interaction energy

  • ΔE_elec: Electrostatic interaction energy

  • ΔG_polar: Polar solvation free energy

  • ΔG_nonpolar: Non-polar solvation free energy

For more detailed information, please refer to the Usage chapter.