Structure Optimization (xTB)

This page covers geometry optimization, single point, and Hessian/frequency workflows using xTB (Extended Tight Binding).

Geometry Optimization

Find a local minimum structure with GFN-xTB / GFN-FF.

chemsmart run [OPTIONS] xtb [XTB_OPTIONS] opt [SUBCMD_OPTIONS]

Optimization Options

Option

Type

Description

-O, --optimization-level

choice

Convergence level: crude, sloppy, loose, lax, normal, tight, vtight, extreme. If not specified, defaults to the xTB default (vtight).

-c, --constrain

string

Coordinates to constrain (1-indexed). Distance [i,j], angle [i,j,k], dihedral [i,j,k,l]. Example: [[1,2],[5,7,8],[3,4,1,7]]

-f, --force-constant

float

Force constant for constraints

Basic Usage

Standard geometry optimization:

chemsmart run xtb -p project -f input.xyz -c 0 -m 1 opt

Override the project optimization level:

chemsmart run xtb -p project -f molecule.xyz opt -O loose

Constrained optimization (writes {label}.inp and passes --input):

chemsmart run xtb -p project -f molecule.xyz opt \
  --constrain '[[1,2],[5,7,8],[3,4,1,7]]' --force-constant 0.5

HPC job submission:

chemsmart sub -s server xtb -p project -f molecule.xyz opt

Solvated Optimization

Solvent options are specified at the xTB group level (before opt). See xTB CLI Options for the full solvent option reference.

# ALPB(toluene)
chemsmart run xtb -p project -f molecule.xyz -sm alpb -si toluene -a solv opt

# Gas phase when the project YAML already sets a solvent
chemsmart run xtb -p solv_project -f molecule.xyz --remove-solvent -a gas opt

Examples

Optimization with GFN1 and gradients:

chemsmart run xtb -p project -f molecule.xyz -g gfn1 --grad opt

Optimization from a Gaussian or ORCA output:

chemsmart sub -s server xtb -p project -f water_opt.out -c 0 -m 1 -l water_xtbopt opt

Single Point Calculations

Compute energy (and optional properties) at a fixed geometry.

chemsmart run [OPTIONS] xtb [XTB_OPTIONS] sp

Basic Usage

chemsmart run xtb -p project -f input.xyz sp

Single point with gradient output and extra xTB flags:

chemsmart run xtb -p project -f molecule.xyz --grad -r "--json" sp

Solvated single point:

chemsmart run xtb -p project -f molecule.xyz -sm alpb -si water sp

Hessian / Frequency Calculations

Compute the numerical Hessian and vibrational frequencies on the input geometry.

chemsmart run [OPTIONS] xtb [XTB_OPTIONS] hess

Basic Usage

chemsmart run xtb -p project -f molecule.xyz hess

Output Layout

Each xTB job runs in a dedicated folder named after the job label. CHEMSMART writes {label}.xyz and redirects stdout/stderr to {label}.out / {label}.err. When -c/--constrain is used, it also writes {label}.inp. Additional xTB artifacts (charges, xtbopt.xyz, g98.out, hessian, etc.) appear in the same folder depending on the job type and flags.

The resulting files and folders can later be:

  • visualized or reused as geometry input (-f path/to/{label}.out)

  • analyzed with Thermochemistry Analysis individually (-f path/to/{label}.out) or in batch (-d path/to/parent_folder + -p xtb)

  • assembled in batch into a database with Assembling Records (-d path/to/parent_folder + -p xtb)

Next Steps