\(\renewcommand{\AA}{\text{Å}}\)

# pair_style pace/apip command

# pair_style pace/fast/apip command

# pair_style pace/precise/apip command

Constant precision variant: pace

## Syntax

    pair_style pace/apip ... keyword values ...
    pair_style pace/fast/apip ... keyword values ...
    pair_style pace/precise/apip ... keyword values ...

- one or more keyword/value pairs may be appended

      keyword = keywords of pair pace

## Examples

    pair_style hybrid/overlay pace/fast/apip pace/precise/apip lambda/input/csp/apip fcc cutoff 5.0 lambda/zone/apip 12.0
    pair_coeff * * pace/fast/apip Cu_fast.yace Cu
    pair_coeff * * pace/precise/apip Cu_precise.yace Cu
    pair_coeff * * lambda/input/csp/apip
    pair_coeff * * lambda/zone/apip

    pair_style hybrid/overlay eam/fs/apip pace/precise/apip lambda/input/csp/apip fcc cutoff 5.0 lambda/zone/apip 12.0
    pair_coeff * * eam/fs/apip Cu.eam.fs Cu
    pair_coeff * * pace/precise/apip Cu_precise.yace Cu
    pair_coeff * * lambda/input/csp/apip
    pair_coeff * * lambda/zone/apip

## Description

Pair style pace computes interactions using the Atomic Cluster Expansion (ACE), which is a general expansion of the atomic energy in multi-body basis functions (Drautz19). The pace pair style provides an efficient implementation that is described in this paper (Lysogorskiy21).

The potential energy \(E_i\) of an atom \(i\) of an adaptive-precision interatomic potential (APIP) according to (Immel25) is given by

\[E_i^\text{APIP} = \lambda_i E_i^\text{(fast)} + (1-\lambda_i) E_i^\text{(precise)}\,,\]

whereas the switching parameter \(\lambda_i\) is computed dynamically during a simulation by fix lambda/apip or set prior to a simulation via set.

The pair style pace/precise/apip computes the potential energy \((1-\lambda_i) E_i^\text{(pace)}\) and the corresponding force and should be combined with a fast potential that computes the potential energy \(\lambda_i E_i^\text{(fast)}\) and the corresponding force via pair_style hybrid/overlay.

The pair style pace/fast/apip computes the potential energy \(\lambda_i E_i^\text{(pace)}\) and the corresponding force and should be combined with a precise potential that computes the potential energy \((1-\lambda_i) E_i^\text{(precise)}\) and the corresponding force via pair_style hybrid/overlay.

The pair_styles pace/fast/apip and pace/precise/apip commands may be followed by the optional keywords of pair_style pace, which are described here.

## Mixing, shift, table, tail correction, restart, rRESPA info

For atom type pairs I,J and I != J, where types I and J correspond to two different element types, mixing is performed by LAMMPS with user-specifiable parameters as described above. You never need to specify a pair_coeff command with I != J arguments for this style.

This pair styles does not support the pair_modify shift, table, and tail options.

This pair styles does not write its information to binary restart files, since it is stored in potential files. Thus, you need to re-specify the pair_style and pair_coeff commands in an input script that reads a restart file.

This pair styles can only be used via the pair keyword of the run_style respa command. It does not support the inner, middle, outer keywords.

## Restrictions

This pair styles are part of the APIP package. It is only enabled if LAMMPS was built with that package. See the Build package page for more info.

## Related commands

pair_style pace, pair_style hybrid/overlay, fix lambda/apip, fix lambda_thermostat/apip, pair_style lambda/zone/apip, pair_style lambda/input/apip, pair_style eam/apip, fix atom_weight/apip

## Default

See pair_style pace.

(Drautz19) Drautz, Phys Rev B, 99, 014104 (2019).

(Lysogorskiy21) Lysogorskiy, van der Oord, Bochkarev, Menon, Rinaldi, Hammerschmidt, Mrovec, Thompson, Csanyi, Ortner, Drautz, npj Comp Mat, 7, 97 (2021).

(Immel25) Immel, Drautz and Sutmann, J Chem Phys, 162, 114119 (2025)