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Interatomic Potential

The interatomic potential is chosen through the compute_force block. By default it does nothing (compute_force: nop); it is invoked as part of compute_all_forces_energy, called at every step of any numerical scheme:

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compute_force_prolog: zero_force_energy
compute_force: nop
compute_force_epilog: force_to_accel

compute_all_forces_energy:
  - compute_force_prolog
  - compute_force
  - compute_force_epilog

Redefining compute_force in your input file is enough to select a potential — you don't need to touch compute_force_prolog/compute_force_epilog or compute_all_forces_energy. Three examples are given below: a simple analytical pair potential with no external file (Lennard-Jones), a tabulated potential read from a file (EAM alloy), and a more complex analytical potential (MEAM).

Lennard-Jones (Argon)

lj_compute_force computes the standard 12/6 Lennard-Jones pair potential from two parameters, epsilon and sigma, up to a cutoff rcut:

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species:
  - Ar: { mass: 39.948 Da, z: 18, charge: 0.0 e- }

lj_compute_force:
  parameters: { epsilon: 0.0104 eV, sigma: 3.4 ang }
  rcut: 8.0 ang

compute_force: lj_compute_force

Tip

For a system with multiple species, lj_multi_force takes a common_parameters fallback plus a per-pair parameters list keyed by type_a/type_b — see Lennard-Jones for details.

EAM alloy (Tantalum)

eam/alloy-format potentials are read from a tabulated file. The eam_alloy_force operator computes the whole potential — electron density, embedding energy and force — in a single pass:

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species:
  - Ta: { mass: 180.95 Da, z: 73, charge: 0 e- }

eam_alloy_force:
  rcut: 5.30 ang
  parameters:
    file: "Ta1_Ravelo_2013.eam.alloy"

compute_force:
  - eam_alloy_force

Tip

More complex scenarios (multi-species alloys, the multi-pass/optimized variant with a separate ghost exchange of the embedding derivative, etc.) can also be set up — see the reference EAM alloy section (still being written).

MEAM (Tin)

meam_force implements the full (2NN) MEAM formalism directly from a set of analytical parameters — no external file:

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species:
  - Tin: { mass: 197.1508269E-27 kg, z: 50, charge: 0 e- }

meam_force:
  rcut: 3.69 ang
  parameters:
    rmax: 3.69 ang
    rmin: 0.0
    Ecoh: 4.93474273599999995322e-19 J
    E0: 4.93474273599999995322e-19 J
    A: 1.000E+00
    r0: 3.44 ang
    alpha: 6.200E+00
    delta: 0.000E+00
    beta0: 6.200E+00
    beta1: 6.000E+00
    beta2: 6.000E+00
    beta3: 6.000E+00
    t0: 1.000E+00
    t1: 4.500E+00
    t2: 6.500E+00
    t3: -0.183E+00
    s0: 1.440E+02
    s1: 0.000E+00
    s2: 0.000E+00
    s3: 0.000E+00
    Cmin: 0.800E+00
    Cmax: 2.800E+00
    Z: 1.200E+01
    rc: 4.0 ang
    rp: 0.1 ang

compute_force: meam_force

Tip

rcut/rmax/rmin/rc/rp are cutoff-related distances, Ecoh/E0/A/r0/alpha/delta describe the universal energy curve, beta0-3/t0-3 control the electron density contributions, s0-3 are screening-related, and Cmin/Cmax/Z set the angular screening function — see the reference MEAM potentials section (still being written) for the full formalism.