# XTBMethod

The XTBMethod object specifies the settings for using Grimme's xTB as the method for a calculation.

xTB is a family of methods based on density functional tight-binding, suitable for the calculation of structures, vibrational frequencies, and non-covalent interactions in large molecular systems. A key feature of these methods is that it almost entirely avoids element-pair-wise parameterisation. The parameterisation consequently covers all spd-block elements and the lanthanides, up to Z = 86, using reference data at the hybrid density functional theory level.

There is one field required for creating an XTBMethod object:

• model for specifying the xTB model (available models are GFN0 and GFN1)

All other calculation details can be specified in the details dictionary.

In case of convergence issues, it is recommended to

• use solver=scc and guess=eeq
• successively decrease the broyden_damping
• successively increase max_iter

This will lead to convergence in almost all cases.

## Example

The following examples demonstrate how to create and use XTBMethod with a SingleInput to calculate the energy of a molecule.

import sierra
from sierra.inputs import *

# Create a water molecule from pubchem (internet connection required)
water = Molecule(pubchem="water")

# Perform a single-point energy calculation using the GFN0-xTB method
xtb0_basic_input = SingleInput(molecule=water, method=XTBMethod(model="GFN0"))
xtb0_basic_result = sierra.run(xtb0_basic_input)
print(f"GFN0-xTB energy: {xtb0_basic_result.energy:.6f} Hartree")
#> GFN0-xTB energy: -4.367421 Hartree

# Perform a single-point energy calculation using the GFN1-xTB method
xtb1_basic_input = SingleInput(molecule=water, method=XTBMethod(model="GFN1"))
xtb1_basic_result = sierra.run(xtb1_basic_input)
print(f"GFN1-xTB energy: {xtb1_basic_result.energy:.6f} Hartree")
#> GFN1-xTB energy: -5.768441 Hartree

# Use more advanced settings for the GFN1-xTB method
molecule=water,
method=XTBMethod(model="GFN1", details={"broyden_damping": 0.2}),
)
#> GFN1-xTB energy: -5.768441 Hartree


### Fields

All of the fields in MethodBase and the following:

model

Specify the xTB model, allowed xTB form are GFN0 and GFN1

• Type: str
• Default: GFN1
details

Dict containing additional detail options. See below.

• Type: Dict[str, Any]

### Available options for details

max_iter

Maximum number of iterations.

• The type is int
• The default is 128
• The value must be nonnegative
energy_threshold

Convergence threshold using the absolute value of the energy difference between iterations.

• The type is float
• The default is 1e-6
• The value must be nonnegative
orbital_grad_threshold

Convergence threshold using the infinity norm of the orbital gradient.

• The type is float
• The default is 1e-5
• The value must be nonnegative
solver

Type of solver to use for determining the density and orbitals. The default is automatically set depending on the xTB model.

• The type is str
• There is no default value.
• The value must be one of:
• scf - Perform a standard self-consistent field calculation
• scc - Perform a self-consistent charge calculation with Broyden mixing
• non_iterative - Perform a non-iterative (single diagonalization) calculation
guess

Initial guess for the xTB wavefunction.

• The type is str
• SAD - Superposition of Atomic Densities (SAD).
• eeq - Use EEQ charges as the initial guess.
broyden_damping
Factor of new shell-resolved charges to be used in Broyden mixing: $$q_{n+1} = f * q_{n} + ...$$ where $$f$$ is the factor.