Restraints Guide

This guide covers how to define and apply distance restraints in PolyzyMD simulations.

Overview

Restraints are commonly used to:

Keep a substrate near the enzyme active site
Maintain specific protein-ligand contacts
Prevent dissociation during equilibration

PolyzyMD supports several restraint types with flexible atom selection.

Restraint Types

Flat-Bottom Restraint

The most common restraint type. No force is applied when atoms are within the threshold distance; a harmonic force kicks in beyond it.

U(r) = 0                      if r < r₀
U(r) = ½k(r - r₀)²           if r ≥ r₀

restraints:
  - type: "flat_bottom"
    name: "substrate_active_site"
    atom1:
      selection: "resid 77 and name OG"
    atom2:
      selection: "resname LIG and name C1"
    distance: 3.3        # Angstroms - threshold
    force_constant: 10000.0  # kJ/mol/nm²
    enabled: true

Harmonic Restraint

Standard harmonic potential - always applies force to maintain target distance.

U(r) = ½k(r - r₀)²

restraints:
  - type: "harmonic"
    name: "distance_restraint"
    atom1:
      selection: "resid 50 and name CA"
    atom2:
      selection: "resid 100 and name CA"
    distance: 10.0       # Angstroms - target distance
    force_constant: 1000.0
    enabled: true

Upper Wall

Prevents distance from exceeding threshold (same as flat-bottom).

- type: "upper_wall"

Lower Wall

Prevents distance from going below threshold.

U(r) = ½k(r₀ - r)²           if r < r₀
U(r) = 0                      if r ≥ r₀

- type: "lower_wall"

Atom Selection Syntax

PolyzyMD uses an MDAnalysis-inspired selection syntax.

Available Keywords

Keyword	Indexing	Description	Example
`resid`	1-indexed	Residue number (matches PDB/PyMOL)	`resid 77`
`resname`	-	Residue name	`resname SER`
`name`	-	Atom name	`name OG`
`index`	0-indexed	OpenMM atom index	`index 2739`
`pdbindex`	1-indexed	PDB atom serial (matches PyMOL)	`pdbindex 2740`
`chain`	-	Chain ID	`chain A`

Combining Selections

Use and / or to combine:

# Both conditions must be true
selection: "resid 77 and name OG"

# Either condition
selection: "resname SER or resname THR"

# Complex combinations
selection: "(resid 77 and name OG) or (resid 110 and name NE2)"

Index Conventions

Important

Understanding indexing is critical for correct restraints!

What you see	Keyword to use	Value to use
PyMOL shows residue 77	`resid`	77
PyMOL shows atom serial 2740	`pdbindex`	2740
OpenMM internal index 2739	`index`	2739

Rule of thumb: Use resid and pdbindex - they match what you see in PyMOL.

Understanding Atom Ordering in PolyzyMD

PolyzyMD builds systems in a consistent, predictable order:

Protein - atoms numbered as in original PDB
Ligand/Substrate - atoms numbered sequentially after protein
Polymers (if enabled) - added after ligand
Solvent and ions - added last

This means if your protein PDB has atoms 1-2722, your ligand atoms will start at 2723.

Important

The solvated_system.pdb file generated by PolyzyMD is your reference for atom indices. Always use this file (not your input structures) when determining pdbindex values.

Workflow: Finding Atom Indices with PyMOL

Step 1: Run Your Simulation Build

First, build your system to generate solvated_system.pdb:

polyzymd run -c config.yaml --replicate 1
# Or just build without running:
polyzymd build -c config.yaml

Step 2: Open the Solvated PDB in PyMOL

pymol /path/to/scratch/dir/solvated_system.pdb

Step 3: Select and Label Your Target Atom

Click on the atom you want to restrain
Right-click the atom in the viewer
Navigate to: Label → atom identifiers → ID

This displays the atom’s ID number directly on the structure - this is the pdbindex value you need.

Tip

You can also see the atom ID in the PyMOL console when you click on an atom:

You clicked /solvated_system//RBY`1/C13x pk1: … id 2739

Step 4: Write Your Selection

Use the displayed ID with pdbindex:

atom2:
  selection: "resname RBY and pdbindex 2739"
  description: "Substrate ester carbonyl carbon"

Real Example: Restraining a Docked Ligand

Here’s a real example from a LipA enzyme simulation with a Resorufin Butyrate substrate.

The Solvated PDB Structure

After building the system, the solvated_system.pdb shows the ligand atoms following the protein:

TER    2722      ASN A 181
HETATM 2723  C1x RBY B   1      72.639  64.759  41.006  1.00  0.00           C  
HETATM 2724  C2x RBY B   1      73.757  65.093  40.238  1.00  0.00           C  
HETATM 2725  C3x RBY B   1      74.974  65.339  40.886  1.00  0.00           C  
HETATM 2726  C4x RBY B   1      75.074  65.248  42.284  1.00  0.00           C  
HETATM 2727  C5x RBY B   1      73.950  64.894  43.027  1.00  0.00           C  
HETATM 2728  C6x RBY B   1      72.739  64.634  42.394  1.00  0.00           C  
HETATM 2729  N1x RBY B   1      76.267  65.488  42.979  1.00  0.00           N  
HETATM 2730  C7x RBY B   1      77.297  65.796  42.258  1.00  0.00           C  
HETATM 2731  C8x RBY B   1      78.582  66.067  42.936  1.00  0.00           C  
HETATM 2732  C9x RBY B   1      79.679  66.376  42.240  1.00  0.00           C  
HETATM 2733 C10x RBY B   1      79.654  66.462  40.768  1.00  0.00           C  
HETATM 2734  O1x RBY B   1      80.663  66.736  40.132  1.00  0.00           O  
HETATM 2735 C11x RBY B   1      78.374  66.201  40.088  1.00  0.00           C  
HETATM 2736 C12x RBY B   1      77.264  65.893  40.770  1.00  0.00           C  
HETATM 2737  O2x RBY B   1      76.065  65.659  40.104  1.00  0.00           O  
HETATM 2738  O3x RBY B   1      71.447  64.422  40.347  1.00  0.00           O  
HETATM 2739 C13x RBY B   1      70.808  63.259  40.777  1.00  0.00           C   <- Ester carbonyl
HETATM 2740  O4x RBY B   1      69.599  63.071  40.828  1.00  0.00           O  
HETATM 2741 C14x RBY B   1      71.835  62.259  41.248  1.00  0.00           C  
...
TER    2757      RBY B   1
HETATM 2758  C1x SBM C   1      80.069  63.041  53.056  1.00  0.00           C   <- Polymers start

The Configuration

To restrain atom C13x (the ester carbonyl carbon, pdbindex 2739) near the catalytic serine:

restraints:
  - type: "flat_bottom"
    name: "substrate_active_site"
    atom1:
      selection: "resid 77 and name OG"
      description: "Catalytic serine oxygen"
    atom2:
      selection: "resname RBY and pdbindex 2739"
      description: "Substrate ester carbonyl carbon"
    distance: 10.0        # Angstroms - no force applied within this distance
    force_constant: 10000.0
    enabled: true

What You’ll See When Running

When the simulation runs, you’ll see confirmation that the restraint was applied:

Applying 1 restraint(s)...
  - substrate_active_site: atom 1192 <-> atom 2738 (type=flat_bottom, d=10.0 Å, k=10000.0 kJ/mol/nm²)
Successfully applied 1 restraint(s)
Running energy minimization...

Note

The logged atom indices are 0-indexed (OpenMM internal). They will be one less than the pdbindex values.

Common Restraint Patterns

Substrate in Active Site

Keep substrate within bonding distance of catalytic residue:

restraints:
  - type: "flat_bottom"
    name: "substrate_catalytic"
    atom1:
      selection: "resid 77 and name OG"
      description: "Catalytic serine"
    atom2:
      selection: "resname LIG and name C1"
      description: "Substrate carbonyl carbon"
    distance: 3.5
    force_constant: 10000.0
    enabled: true

Multiple Active Site Contacts

restraints:
  - type: "flat_bottom"
    name: "contact_1"
    atom1:
      selection: "resid 77 and name OG"
    atom2:
      selection: "resname LIG and name C1"
    distance: 3.5
    force_constant: 10000.0
    enabled: true
    
  - type: "flat_bottom"
    name: "contact_2"
    atom1:
      selection: "resid 110 and name NE2"
    atom2:
      selection: "resname LIG and name O2"
    distance: 3.5
    force_constant: 5000.0
    enabled: true
    
  - type: "flat_bottom"
    name: "contact_3"
    atom1:
      selection: "resid 12 and name N"
    atom2:
      selection: "resname LIG and name O1"
    distance: 4.0
    force_constant: 5000.0
    enabled: false  # Disabled for now

Protein-Protein Distance

Monitor or restrain distance between protein regions:

restraints:
  - type: "harmonic"
    name: "domain_distance"
    atom1:
      selection: "resid 50 and name CA"
    atom2:
      selection: "resid 150 and name CA"
    distance: 25.0
    force_constant: 100.0  # Weak restraint
    enabled: true

Force Constant Guidelines

Use Case	Force Constant (kJ/mol/nm²)
Strong restraint (prevent dissociation)	10000 - 50000
Moderate restraint	1000 - 5000
Weak/guiding restraint	100 - 500
Very weak (biasing)	10 - 50

Note

Start with stronger restraints during equilibration, then reduce or disable for production if desired.

Troubleshooting

“No atoms match selection”

ValueError: No atoms match selection: 'resid 77 and name OG'

Causes:

Residue numbering doesn’t match (check PDB)
Atom name is different (check PDB atom names)
Typo in selection string

Solution: Open PDB in PyMOL and verify exact residue numbers and atom names.

“Requires exactly one atom per selection”

ValueError: Restraint 'my_restraint' requires exactly one atom per selection.
Got 5 for atom1, 1 for atom2

Cause: Selection matches multiple atoms.

Solution: Make selection more specific:

# Too broad - matches all atoms in residue
selection: "resid 77"

# Better - matches specific atom
selection: "resid 77 and name OG"

# Most specific - use atom index
selection: "resid 77 and pdbindex 1193"

Restraint Not Taking Effect

Check enabled: true is set
Verify distance units (should be Angstroms)
Check force constant is reasonable (not too small)
Verify atom selections resolve correctly

Disabling Restraints

In Configuration

restraints:
  - type: "flat_bottom"
    name: "my_restraint"
    # ... other settings ...
    enabled: false  # Disabled

No Restraints at All

restraints: []

Or simply omit the restraints section entirely.