\(\renewcommand{\AA}{\text{Å}}\)

# pair_style lambda/input/apip command

## Syntax

    pair_style lambda/input/apip cutoff

- lambda/input/apip = style name of this pair style

- cutoff = global cutoff (distance units)

# pair_style lambda/input/csp/apip command

## Syntax

    pair_style lambda/input/csp/apip lattice keyword args

- lambda/input/csp/apip = style name of this pair style

- lattice = fcc or bcc or integer

      fcc = use 12 nearest neighbors to calculate the CSP like in a perfect fcc lattice
      bcc = use 8 nearest neighbors to calculate the CSP like in a perfect bcc lattice
      integer = use N nearest neighbors to calculate the CSP

- zero or more keyword/args pairs may be appended

- keyword = cutoff or N_buffer

      cutoff args = cutoff
        cutoff = distance in which neighboring atoms are considered (> 0)
      N_buffer args = N_buffer
        N_buffer = number of additional neighbors, which are included in the j-j+N/2 calculation

## Examples

    pair_style lambda/input/csp/apip fcc
    pair_style lambda/input/csp/apip fcc cutoff 5.0
    pair_style lambda/input/csp/apip bcc cutoff 5.0 N_buffer 2
    pair_style lambda/input/csp/apip 14

## Description

This pair_styles calculates \(\lambda_i^\text{input}(t)\), which is required for fix lambda/apip.

The pair_style lambda_input sets \(\lambda_i^\text{input}(t) = 0\).

The pair_style lambda_input/csp calculates \(\lambda_i^\text{input}(t) = \text{CSP}_i(t)\). The centro-symmetry parameter (CSP) (Kelchner) is described in compute centro/atom.

The lattice argument is described in compute centro/atom and determines the number of neighboring atoms that are used to compute the CSP. The N_buffer argument allows to include more neighboring atoms in the calculation of the contributions from the pair j,j+N/2 to the CSP as discussed in (Immel).

The computation of \(\lambda_i^\text{input}(t)\) is done by this pair_style instead of by fix lambda/apip, as this computation takes time and this pair_style can be included in the load-balancing via fix atom_weight/apip.

A code example for the calculation of the switching parameter for an adaptive- precision potential is given in the following: The adaptive-precision potential is created by combining pair_style eam/fs/apip and pair_style pace/precise/apip. The input, from which the switching parameter is calculated, is provided by this pair_style. The switching parameter is calculated by fix lambda/apip, whereas the spatial transition zone of the switching parameter is calculated by pair_style 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
    fix 2 all lambda/apip 3.0 3.5 time_averaged_zone 4.0 12.0 110 110 min_delta_lambda 0.01

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

The cutoff distance for this pair style can be mixed. The default mix value is geometric. See the “pair_modify” command for details.

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

This pair style writes no information to binary restart files, so pair_style and pair_coeff commands need to be specified in an input script that reads a restart file.

This pair style does not support the use of the inner, middle, and outer keywords of the run_style respa command.

## Restrictions

This fix is 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

compute centro/atom, fix lambda/apip, fix lambda_thermostat/apip, pair_style lambda/zone/apip, pair_style eam/apip, pair_style pace/apip, fix atom_weight/apip

## Default

N_buffer=0, cutoff=5.0

(Kelchner) Kelchner, Plimpton, Hamilton, Phys Rev B, 58, 11085 (1998).

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