Configuration Reference

This document describes all configuration options for PolyzyMD YAML files.

Configuration Structure

A complete configuration file has these sections:

name: "simulation_name"
description: "optional description"

enzyme: { ... }           # Required
substrate: { ... }        # Optional (null for apo)
polymers: { ... }         # Optional (null to disable)
solvent: { ... }          # Required
restraints: [ ... ]       # Optional
thermodynamics: { ... }   # Required
simulation_phases: { ... } # Required
output: { ... }           # Required
force_field: { ... }      # Optional (has defaults)

---

Enzyme Configuration

enzyme:
  name: "LipA"                           # Identifier (required)
  pdb_path: "structures/enzyme.pdb"      # Path to PDB file (required)
  description: "Bacillus subtilis Lipase A"  # Optional description

Field	Type	Required	Description
name	string	Yes	Short identifier for the enzyme
pdb_path	path	Yes	Path to prepared PDB file
description	string	No	Human-readable description

---

Substrate Configuration

substrate:
  name: "Resorufin-Butyrate"             # Identifier (required)
  sdf_path: "structures/substrate.sdf"   # Path to SDF file (required)
  conformer_index: 0                     # Which conformer to use (default: 0)
  charge_method: "nagl"                  # Charge assignment method
  residue_name: "LIG"                    # 3-letter residue name

Field	Type	Required	Default	Description
name	string	Yes	-	Substrate identifier
sdf_path	path	Yes	-	Path to SDF with docked conformers
conformer_index	int	No	0	Index of conformer to use (0-indexed)
charge_method	string	No	“nagl”	Options: nagl, espaloma, am1bcc
residue_name	string	No	“LIG”	3-letter code for topology

Charge Methods

Method	Description	Speed
nagl	Graph neural network charges	Fast
espaloma	Machine learning charges	Medium
am1bcc	Semi-empirical QM charges	Slow

No Substrate (Apo Simulation)

substrate: null

---

Polymer Configuration

PolyzyMD supports two modes for polymer generation: cached (load from pre-built SDF files) and dynamic (generate on-the-fly from SMILES).

Tip

For a complete guide on dynamic polymer generation, see Dynamic Polymer Generation.

Basic Configuration (Cached Mode)

polymers:
  enabled: true                          # Enable/disable polymers
  type_prefix: "SBMA-EGPMA"              # Polymer type identifier
  
  monomers:                              # Monomer definitions
    - label: "A"                         # Single character label
      probability: 0.98                  # Selection probability (0-1)
      name: "SBMA"                       # Full name (optional)
    - label: "B"
      probability: 0.02
      name: "EGPMA"
  
  length: 5                              # Monomers per chain
  count: 2                               # Number of polymer chains
  
  sdf_directory: null                    # Pre-built polymer SDFs (optional)
  cache_directory: ".polymer_cache"      # Cache for generated polymers

Dynamic Generation Configuration

To generate polymers on-the-fly from monomer SMILES (without pre-built SDF files):

polymers:
  enabled: true
  generation_mode: "dynamic"             # Enable dynamic generation
  type_prefix: "SBMA-EGPMA"
  
  # ATRP reaction templates (use bundled defaults or custom paths)
  reactions:
    initiation: "default"                # or "/path/to/custom.rxn"
    polymerization: "default"
    termination: "default"
  
  monomers:
    - label: "A"
      probability: 0.7
      name: "SBMA"
      smiles: "[H]C([H])=C(C(=O)OC...)..."  # Required for dynamic mode
      residue_name: "SBM"                   # Optional 3-letter residue name
    - label: "B"
      probability: 0.3
      name: "EGPMA"
      smiles: "[H]C([H])=C(C(=O)OC...)..."
      residue_name: "EGM"
  
  length: 5
  count: 2
  charger: "nagl"                        # Charge method: nagl, espaloma, am1bcc
  max_retries: 10                        # Retries for ring-piercing detection
  cache_directory: ".polymer_cache"

All Polymer Options

Field	Type	Required	Default	Description
enabled	bool	No	true	Enable polymer addition
generation_mode	string	No	“cached”	cached or dynamic
type_prefix	string	Yes	-	Identifier for polymer type
monomers	list	Yes	-	Monomer specifications
length	int	Yes	-	Chain length (number of monomers)
count	int	Yes	-	Number of chains to add
sdf_directory	path	No	null	Directory with pre-built polymer SDFs
cache_directory	path	No	“.polymer_cache”	Cache directory
reactions	object	No	all “default”	ATRP reaction templates (dynamic mode)
charger	string	No	“nagl”	Charge method for dynamic generation
max_retries	int	No	10	Max attempts for ring-piercing avoidance

Monomer Specification

Field	Type	Required	Description
label	string	Yes	Single character (A, B, C…)
probability	float	Yes	Selection probability (must sum to 1.0)
name	string	No	Full monomer name
smiles	string	Dynamic only	Raw monomer SMILES (with C=C double bond)
residue_name	string	No	3-letter residue code for topology

Charge Methods for Dynamic Generation

Method	Description	Speed	Accuracy
nagl	Graph neural network charges	Fast	Good
espaloma	Machine learning charges	Medium	Good
am1bcc	Semi-empirical QM charges	Slow	Best

No Polymers

polymers: null
# or
polymers:
  enabled: false

---

Solvent Configuration

solvent:
  primary:
    type: "water"
    model: "tip3p"                       # Water model
  
  co_solvents: []                        # List of co-solvents (optional)
  
  ions:
    neutralize: true                     # Add counter-ions
    nacl_concentration: 0.15             # NaCl concentration (M)
  
  box:
    padding: 1.2                         # nm from solute to box edge
    shape: "rhombic_dodecahedron"        # Box shape
    target_density: 1.0                  # g/mL
    tolerance: 2.0                       # PACKMOL tolerance (Angstrom)

Water Models

Model	Description
tip3p	TIP3P (default, fast)
spce	SPC/E
tip4pew	TIP4P-Ew
opc	OPC (accurate, slower)

Box Shapes

Shape	Description
cube	Cubic box
rhombic_dodecahedron	Space-efficient (default)
truncated_octahedron	Alternative space-efficient

Co-solvents

PolyzyMD supports adding co-solvents to your simulation system. You can specify co-solvents using either volume fraction (v/v) or molar concentration.

Specification Methods

Method	Field	Description	Effect on Water
Volume Fraction	volume_fraction	Fraction of box volume (0-1)	Reduces water proportionally
Concentration	concentration	Molar concentration (mol/L)	Additive (water unchanged)

Important: Use exactly ONE method per co-solvent. Do not specify both volume_fraction and concentration for the same co-solvent.

Volume Fraction Method

Use this when you want a specific percentage of the solvent to be the co-solvent (e.g., “30% DMSO”).

co_solvents:
  - name: "dmso"
    volume_fraction: 0.30    # 30% v/v DMSO

Formula:

n = (V_box × phi × rho) / M

Where:
  n     = number of co-solvent molecules
  V_box = simulation box volume (L)
  phi   = volume fraction (e.g., 0.30 for 30%)
  rho   = co-solvent density (g/mL)
  M     = molar mass (g/mol)

Source: src/polyzymd/builders/solvent.py:267-287

The water count is reduced proportionally: if you specify 30% DMSO, water fills the remaining 70% of the box.

Concentration Method

Use this when you want a specific molar concentration (e.g., “2 M urea for protein denaturation studies”).

co_solvents:
  - name: "urea"
    concentration: 2.0       # 2 M urea

Formula:

n = C × V_box × N_A

Where:
  n     = number of co-solvent molecules
  C     = concentration (mol/L)
  V_box = simulation box volume (L)
  N_A   = Avogadro's number (implicit in OpenMM)

Source: src/polyzymd/builders/solvent.py:295-312

The water count is NOT reduced when using concentration. The co-solvent molecules are added to the existing water, which may slightly increase the effective density.

Built-in Co-solvent Library

PolyzyMD includes a library of common co-solvents with pre-defined SMILES and densities. Density values are sourced from PubChem, a public database of chemical compounds. Each compound has a unique Compound Identification Number (CID) that can be used to look up detailed information including density, structure, and safety data.

Name	SMILES	Density (g/mL)	Reference
dmso	CS(=O)C	1.10	CID 679
dmf	CN(C)C=O	0.95	CID 6228
acetonitrile	CC#N	0.786	CID 6342
urea	C(=O)(N)N	1.32	CID 1176
ethanol	CCO	0.789	CID 702
methanol	CO	0.792	CID 887
glycerol	C(C(CO)O)O	1.261	CID 753
isopropanol	CC(C)O	0.786	CID 3776
acetone	CC(=O)C	0.784	CID 180
thf	C1CCOC1	0.883	CID 8028
dioxane	C1COCCO1	1.033	CID 31275
ethylene_glycol	C(CO)O	1.114	CID 174

For library co-solvents, you only need to specify the name and either volume_fraction or concentration:

co_solvents:
  - name: "dmso"
    volume_fraction: 0.10    # 10% v/v DMSO - smiles and density auto-populated

Custom Co-solvents

For molecules not in the library, you must provide the SMILES string. Density is required only when using volume_fraction:

co_solvents:
  # Custom co-solvent with volume fraction (density required)
  - name: "ethyl_acetate"
    smiles: "CCOC(=O)C"
    density: 0.902           # g/mL - required for volume_fraction
    volume_fraction: 0.15

  # Custom co-solvent with concentration (density not needed)
  - name: "my_additive"
    smiles: "CC(=O)NC"
    concentration: 0.5       # 0.5 M

Multiple Co-solvents

You can combine multiple co-solvents. Each can use either specification method independently:

co_solvents:
  - name: "dmso"
    volume_fraction: 0.20    # 20% v/v DMSO
  - name: "urea"
    concentration: 1.0       # Plus 1 M urea

Warning: When using multiple co-solvents with volume_fraction, ensure the total does not exceed 1.0 (100%). The remaining fraction is filled with water.

Warning

YAML List Syntax

A common mistake is placing each field on a separate line with its own -, which creates multiple list items instead of one object with multiple fields.

Incorrect (creates 3 separate incomplete items):

co_solvents:
  - name: "dmso"
  - volume_fraction: 0.30
  - residue_name: "DMS"

Correct (one item with 3 fields):

co_solvents:
  - name: "dmso"
    volume_fraction: 0.30
    residue_name: "DMS"

The - character starts a new list item. All fields belonging to the same item must be indented to the same level without a leading -.

Assumptions and Limitations

• Ideal mixing: Volume fractions assume ideal mixing (volumes are additive). Real solutions may deviate.

• Room temperature densities: Library densities are approximate values at ~25C.

• PACKMOL placement: Co-solvent molecules are placed randomly by PACKMOL and may require equilibration to achieve uniform distribution.

Solvent Parameterization

PolyzyMD uses pre-computed partial charges for all solvent molecules to ensure consistency and performance.

Why Pre-computed Charges?

When adding many copies of the same solvent molecule (e.g., 1000 DMSO molecules), each molecule should have identical partial charges. However, charge calculation methods like AM1BCC have numerical variability - running the calculation twice on the same molecule can produce slightly different charges.

If charges were computed independently for each solvent molecule:

1. Inconsistency: Identical molecules would have different parameters (physically incorrect)

2. Performance: AM1BCC is expensive; computing it 1000x is wasteful

3. Force field issues: Parameter variability can cause OpenFF Interchange errors

How It Works

PolyzyMD solves this by computing charges once and reusing them:

1. Built-in solvents: Pre-computed SDF files are bundled with the package (in src/polyzymd/data/solvents/)

2. User cache: Custom solvents are cached in ~/.polyzymd/solvent_cache/ after first use

3. Lookup order: Memory cache → Bundled SDFs → User cache → Generate and cache

# Lookup order for get_solvent_molecule("dmso")
1. Check in-memory cache (fastest)
2. Check bundled library: src/polyzymd/data/solvents/dmso.sdf
3. Check user cache: ~/.polyzymd/solvent_cache/dmso.sdf
4. Generate from SMILES + AM1BCC, save to user cache

Available Pre-computed Solvents

All 12 library co-solvents plus water models have pre-computed charges:

Solvent	File	Charge Method
TIP3P Water	tip3p.sdf	Literature values
DMSO	dmso.sdf	AM1BCC
DMF	dmf.sdf	AM1BCC
Acetonitrile	acetonitrile.sdf	AM1BCC
Urea	urea.sdf	AM1BCC
Ethanol	ethanol.sdf	AM1BCC
Methanol	methanol.sdf	AM1BCC
Glycerol	glycerol.sdf	AM1BCC
Isopropanol	isopropanol.sdf	AM1BCC
Acetone	acetone.sdf	AM1BCC
THF	thf.sdf	AM1BCC
Dioxane	dioxane.sdf	AM1BCC
Ethylene Glycol	ethylene_glycol.sdf	AM1BCC

Custom Solvents

When you use a custom co-solvent (not in the library), PolyzyMD will:

1. Generate the molecule from your SMILES string

2. Compute AM1BCC partial charges (this may take a few seconds)

3. Cache the parameterized molecule to ~/.polyzymd/solvent_cache/

4. Reuse the cached version for all future simulations

co_solvents:
  - name: "my_custom_solvent"
    smiles: "CC(=O)OCC"        # First use: computes and caches charges
    concentration: 0.5         # Future uses: loads from cache instantly

Managing the Cache

You can inspect and manage the solvent cache programmatically:

from polyzymd.data import list_available_solvents, clear_cache

# List all available solvents (bundled + cached)
solvents = list_available_solvents()
print(solvents)
# {'bundled': ['dmso', 'ethanol', ...], 'cached': ['my_custom_solvent']}

# Clear the user cache (does not affect bundled solvents)
clear_cache()

The user cache location is ~/.polyzymd/solvent_cache/. You can safely delete this directory to force re-computation of custom solvents.

---

Restraints Configuration

restraints:
  - type: "flat_bottom"                  # Restraint type
    name: "substrate_active_site"        # Identifier
    atom1:
      selection: "resid 77 and name OG"  # First atom selection
      description: "Catalytic serine"    # Optional description
    atom2:
      selection: "resname LIG and name C1"
      description: "Substrate carbon"
    distance: 3.3                        # Angstroms
    force_constant: 10000.0              # kJ/mol/nm²
    enabled: true                        # Enable/disable

See Restraints Guide for detailed selection syntax.

Restraint Types

Type	Description
flat_bottom	No force within threshold, harmonic beyond
harmonic	Harmonic potential at target distance
upper_wall	Prevent distance exceeding threshold
lower_wall	Prevent distance below threshold

---

Thermodynamics Configuration

thermodynamics:
  temperature: 300.0                     # Kelvin
  pressure: 1.0                          # atmospheres

---

Simulation Phases Configuration

simulation_phases:
  equilibration:
    ensemble: "NVT"                      # NVT, NPT, or NVE
    duration: 1.0                        # nanoseconds
    samples: 100                         # frames to save
    time_step: 2.0                       # femtoseconds
    thermostat: "LangevinMiddle"
    thermostat_timescale: 1.0            # picoseconds
  
  production:
    ensemble: "NPT"
    duration: 100.0                      # nanoseconds total
    samples: 2500                        # total frames
    time_step: 2.0
    thermostat: "LangevinMiddle"
    thermostat_timescale: 1.0
    barostat: "MC"                       # Monte Carlo barostat
    barostat_frequency: 25               # steps between barostat moves
  
  segments: 10                           # Split production into segments

Ensembles

Ensemble	Description
NVT	Constant volume, temperature
NPT	Constant pressure, temperature
NVE	Microcanonical (no thermostat)

Thermostats

Thermostat	Description
LangevinMiddle	Langevin integrator (recommended)
Langevin	Standard Langevin
Andersen	Andersen thermostat
NoseHoover	Nosé-Hoover chain

Barostats

Barostat	Description
MC	Monte Carlo barostat (recommended)
MCA	Monte Carlo anisotropic

---

Output Configuration

Environment variables ($USER, $HOME, ${VAR}) and ~ are automatically expanded in path fields.

output:
  # Directory structure - environment variables are expanded automatically
  projects_directory: "/projects/$USER/polyzymd"   # Scripts, logs
  scratch_directory: "/scratch/alpine/$USER/simulations"  # Trajectories
  
  # You can also use ~ for home directory
  # projects_directory: "~/polyzymd"
  
  # Subdirectories within projects_directory
  job_scripts_subdir: "job_scripts"
  slurm_logs_subdir: "slurm_logs"
  
  # Naming
  naming_template: "{enzyme}_{substrate}_{polymer_type}_{temperature}K_run{replicate}"
  
  # Output options
  save_checkpoint: true                  # Save restart files
  save_state_data: true                  # Save energy/temperature CSV
  trajectory_format: "dcd"               # dcd or xtc

Naming Template Variables

Variable	Description	Example
{enzyme}	Enzyme name	“LipA”
{substrate}	Substrate name	“ResorufinButyrate”
{polymer_type}	Polymer type	“SBMA-EGPMA”
{temperature}	Temperature in K	“300”
{replicate}	Replicate number	“1”

---

Force Field Configuration

force_field:
  protein: "ff14sb_off_impropers_0.0.4.offxml"  # Protein force field
  small_molecule: "openff-2.0.0.offxml"          # Ligand/polymer force field

Available Force Fields

Protein:

• ff14sb_off_impropers_0.0.4.offxml - Amber ff14SB (recommended)

Small Molecule:

• openff-2.0.0.offxml - OpenFF Sage 2.0 (recommended)

• openff-2.1.0.offxml - OpenFF Sage 2.1

Key Collision Warnings

When building systems with both proteins and small molecules, you may see warnings like:

Key collision with different parameters, fixing. Key is [#6X4:1]-[#1:2]

This is expected behavior and does not indicate a problem.

Why This Happens

PolyzyMD uses different force fields for different molecule types:

• Proteins: ff14SB (Amber force field ported to OpenFF format)

• Small molecules: OpenFF Sage 2.0 (general small molecule force field)

When these force fields are combined, the same SMIRKS pattern (e.g., [#6X4:1]-[#1:2] for sp³ carbon-hydrogen bonds) may appear in both, but with different parameter values. This is expected because:

1. ff14SB was optimized for protein behavior

2. OpenFF Sage was optimized for general organic molecules

3. Both are valid parameterizations for their respective domains

How OpenFF Handles This

OpenFF Interchange detects these collisions and resolves them by appending _DUPLICATE to the key, allowing both parameter sets to coexist:

# Simplified OpenFF behavior
if key in existing_parameters:
    if parameters_are_identical:
        pass  # No action needed
    else:
        key.id += "_DUPLICATE"  # Keep both parameter sets

This ensures that:

• Protein atoms use ff14SB parameters

• Small molecule atoms use OpenFF Sage parameters

• The simulation runs correctly with appropriate parameters for each molecule type

What You’ll See in Logs

With PolyzyMD’s logging, you can identify which molecule combinations trigger collisions:

Combining 7 component Interchange(s)
  Components: LipA, ResorufinButyrate, EGPMA-SBMA_AAABA, ..., dmso, water/ions
[DEBUG] Combining 'LipA' with 'ResorufinButyrate'...
Key collision with different parameters, fixing. Key is [#6X4:1]-[#1:2]
...

Collisions typically occur when combining protein Interchanges (using ff14SB) with small molecule Interchanges (using OpenFF Sage).

Further Reading

For more details on this behavior, see the OpenFF Interchange documentation:

• Sharp Edges: Combining Interchanges

---

Complete Example

See the example configurations in src/polyzymd/configs/examples/:

• enzyme_only.yaml - Enzyme + substrate, no polymers

• enzyme_polymer.yaml - Full enzyme + polymer simulation

• enzyme_cosolvent.yaml - Enzyme with DMSO co-solvent

---

See Also

• Dynamic Polymer Generation - Dynamic polymer generation from SMILES

• GROMACS Export and Simulation - Running simulations with GROMACS

• Polymer Setup Guide - Polymer setup guide

• Restraints Guide - Atom selection and restraints

• CLI Reference - CLI documentation