samgen

Build self-assembled monolayer (SAM) surfaces as GROMACS .gro / .top files.

samgen's job is to place strands correctly on a periodic Au(111) grid per a chosen design, for one-sided and two-sided (shared-sulfur midplane) monolayers. Orientation/tilt and topology assembly are provided as conveniences that you must verify before running MD.

What it does

• Hexagonal Au(111) lattice — (√3×√3)R30° tiling with the staggered-row offset, periodic-correct box.
• SAM orientation — apply the tilt (alpha ≈ 28°, beta ≈ 53°; Love et al. 2005) with an always-on orientation sanity check and opt-in, chemistry-anchored canonicalization.
• Five placement designs — density (default), uniform, two-component grid, fraction/random (seeded), and multi-ligand (3+).
• Anchor / cap detection — prompt-first with consent-to-guess.
• Topology assembly — residue reordering, merged [defaults]/[atomtypes], per-component bonded params, and a [molecules] block with correct counts, optionally gated by gmx grompp.
• Periodicity check — verifies the built surface tiles seamlessly under PBC.
• Two-sided SAMs — fuse two arms onto a single shared sulfur via a proper, chirality-preserving 180° rotation; emits geometry for you to parameterize.

Install

Requires Python 3.11+. Dependencies (numpy, scipy, pyyaml) install automatically.

# from PyPI
pip install samgen

# or the latest from GitHub
pip install git+https://github.com/max3925vats/samgen.git

# or editable, for development (adds pytest)
git clone https://github.com/max3925vats/samgen.git
cd samgen
pip install -e ".[dev]"

GROMACS is optional — it's only used for the gmx grompp topology check (--validate) and any boxing/centering you do yourself. Install it however you like (e.g. brew install gromacs, conda, or your cluster module).

Quick start

The default design places a methyl-terminated (ch3) ligand on a hydroxyl-terminated (coh) matrix at a target areal density:

samgen build configs/ch3_density.yaml -o sam.gro --top topol.top
# -> 132 CH3 ligands on a COH matrix (528 strands), with topol.top

Three stages (run together or separately)

All stages share one config, so you can run them piecewise or in one shot:

# 1) tile the surface geometry (no force field needed)
samgen geometry configs/ch3_grid.yaml -o sam.gro

# 2) assemble the topology for that surface (--validate runs the gmx grompp gate)
samgen topology configs/ch3_grid.yaml --gro sam.gro -o topol.top --validate

# ...or do both at once:
samgen build    configs/ch3_grid.yaml -o sam.gro --top topol.top

# two-sided (shared-S) strand + geometry-only surface:
samgen twosided configs/ch3_grid.yaml --component ch3 -o strand.gro --surface twosided-sam.gro

For twosided, the anchor is prompt-first: it uses the config's anchor: if set, otherwise prompts in an interactive terminal and only auto-detects with your consent. In batch (no TTY) it requires anchor: in the config unless allow_anchor_autodetect: true.

From Python:

from samgen import generate_geometry, assemble_topology, build

Placement designs

A design decides which component sits at each Au(111) lattice site. Set it in the design: block of the config.

• density (default) — ligands at a target areal density (ligands/nm²), placed on a uniform sub-lattice of the gold sites. samgen keeps the grid perfectly periodic; the realized density is quantized to the Au spacing and is reported. If your box and density don't tile uniquely, samgen offers two periodic choices (a smaller box with fewer ligands, or a larger box with more) and you pick — or set density_choice: below|above for batch runs.

  design: {type: density, base: coh, ligand: ch3, density: 1.0}
  

  Example: density 1.0/nm² in a ~10×10 nm box → a 22×24 Au grid, stride 2 → 11×12 = 132 ligands (box 10.978×10.368 nm in rounded-lattice mode).

  For full manual control, ligand_grid: [nx, ny] places exactly nx×ny ligands (the box is grown so the grid stays periodic; per-axis spacing may differ, so anisotropic grids are allowed). It overrides density when both are set.

• fraction — random ligands at a target fraction (0–1), seeded for reproducibility.

  design: {type: fraction, base: coh, ligand: ch3, fraction: 0.25, seed: 42}
  

• uniform — one component on every site.

  design: {type: uniform, component: ch3}
  

• grid — two components from a 0/1 pattern file.

  design: {type: grid, pattern: inputs/pattern.dat, mapping: {0: coh, 1: ch3}}
  

• multilig — 3+ components from an integer pattern file.

The config box: {x, y} sets the surface area; the lattice fills it and snaps x,y to whole Au cells for periodicity. The pattern file (for grid/multilig) does not set the box — it is sampled per site and padded with the base component beyond its extent.

Lattice constants

The default Au(111) spacing is the exact colsep = √3·a (a = 0.288 nm). Set lattice: {rounded: true} to use rounded constants (colsep = 0.499) when you need to reproduce a specific box exactly (e.g. 10.978 × 10.368 nm); ligand counts are identical either way.

Periodicity check

Every build is checked for periodicity under PBC (on by default, warn-only): the box is a whole Au-lattice multiple, the ligand grid has no seam (uniform nearest-neighbour spacing including the wrap), and no two strands fall closer than min_spacing across the periodic boundary. Configure or disable it:

periodicity_check: {enabled: true, on_failure: warn, min_spacing: 0.40, tol: 0.002}

Set on_failure: error to make a non-periodic surface a hard failure instead of a warning.

Input files

The examples ship with ready-to-run strands under configs/inputs/: a methyl-terminated (ch3) and a hydroxyl-terminated (coh) alkanethiol, each as a single-strand .gro (pre-oriented along z) plus a self-contained .itp, and a design pattern.dat for the grid. So configs/ch3_density.yaml, configs/ch3_onesided.yaml, and configs/ch3_grid.yaml run out of the box.

To use your own molecules, drop a single-strand .gro (canonically oriented along z, or pre-tilted with apply_tilt: false) and a matching .itp/.top into configs/inputs/ and point the config's component paths at them.

You finish run-readiness

samgen builds the geometry and assembles a topology, but deliberately leaves two simulation-setup steps to you (it prints a reminder when you assemble a topology):

1. Position restraints — restrain the anchor sulfur (and typically the 7th chain carbon) so the SAM stays anchored. Add a [ position_restraints ] block / #ifdef POSRES to each component .itp and define POSRES in your .mdp.
2. Center / box the structure — e.g. gmx editconf -f sam.gro -o centered.gro -c -d <pad> before solvation/equilibration.

Orientation applicability

samgen derives a strand's tilt/twist frame from the anchor sulfur and the first N backbone carbons of a linear alkyl spacer (default N=9, set per component with backbone_carbons). This matches the canonical alkanethiol-on-Au(111) SAM motif. Strands that depart from it — non-thiol anchors; branched, short, rigid, or aromatic spacers; or headgroups attached within the first N carbons — may not be oriented correctly. In those cases, supply a pre-oriented strand (apply_tilt: false) or verify and correct the generated frame yourself. samgen does not guarantee orientation for arbitrary strand chemistries.

Surface types

samgen places strands on a periodic Au(111) grid — that placement is the guarantee. One-sided and two-sided (shared-sulfur midplane) surfaces are supported. Orientation and topology assembly are provided as conveniences; verify both before running MD (see "You finish run-readiness" and "Orientation applicability" above).

Two-sided SAMs

The tool builds the geometry of a two-sided (shared-S) strand and surface. Because forming the shared sulfur removes the methyl caps and changes local charges, you parameterize the generated strand yourself (acpype/RESP), then hand the .itp/.top back to assemble the integrated topology.

License

BSD-3-Clause — see LICENSE.