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### pair_style comb command

Syntax:

    pair_style comb

Examples:

    pair_style comb
    pair_coeff * * ../potentials/ffield.comb Si
    pair_coeff * * ../potentials/ffield.comb Hf Si O

Description:

Style comb computes a variable charge COMB (Charge-Optimized Many-Body) potential as described in (COMB_1) and (COMB_2). The energy E of a system of atoms is given by

where ET is the total potential energy of the system, ESi is the self-energy term of atom i, Vij is the interatomic potential between the ith and jth atoms, rij is the distance of the atoms i and j, and qi and qj are charges of the atoms, and EBBi is the bond-bending term of atom i.

The interatomic potential energy Vij consists of four components: two-body short-range repulsion, URij, many-body short-range attraction, UAij, long-range Coulombic electrostatic interaction, UIij, and van der Waals energy, UVij, which are defined as:

The short-range repulsion and attraction are based on the Tersoff potential (see the pair_style tersoff command); thus for a zero-charge pure element system with no van der Waals interaction, the COMB potential reduces to Tersoff potential, typically truncated at a short cutoff, e.g. 3 to 4 Angstroms. The long-range Coulombic term uses the Wolf summation method described in Wolf, spherically truncated at a longer cutoff, e.g. 12 Angstroms.

The COMB potential is a variable charge potential. The equilibrium charge on each atom is calculated by the electronegativity equalization (QEq) method. See Rick for further details. This is implemented by the fix qeq/comb command, which should normally be specified in the input script when running a model with the COMB potential. The fix qeq/comb command has options that determine how often charge equilibration is performed, its convergence criterion, and which atoms are included in the calculation.

Only a single pair_coeff command is used with the comb style which specifies the COMB potential file with parameters for all needed elements. These are mapped to LAMMPS atom types by specifying N additional arguments after the potential file in the pair_coeff command, where N is the number of LAMMPS atom types. The provided potential file ffield.comb contains all currently-available COMB parameterizations: for Si, Cu, Hf, Ti, O, their oxides and Zr, Zn and U metals.

For example, if your LAMMPS simulation of a Si/SiO2/ HfO2 interface has 4 atom types, and you want the 1st and last to be Si, the 2nd to be Hf, and the 3rd to be O, and you would use the following pair_coeff command:

    pair_coeff * * ../potentials/ffield.comb Si Hf O Si

The first two arguments must be * * so as to span all LAMMPS atom types. The first and last Si arguments map LAMMPS atom types 1 and 4 to the Si element in the ffield.comb file. The second Hf argument maps LAMMPS atom type 2 to the Hf element, and the third O argument maps LAMMPS atom type 3 to the O element in the potential file. If a mapping value is specified as NULL, the mapping is not performed. This can be used when a comb potential is used as part of the hybrid pair style. The NULL values are placeholders for atom types that will be used with other potentials.

The ffield.comb potential file is in the potentials directory of the LAMMPS distribution. Lines that are not blank or comments (starting with #) define parameters for a triplet of elements. The 49 parameters in a single entry correspond to coefficients in the formula above:

- element 1 (the center atom in a 3-body interaction)
- element 2 (the atom bonded to the center atom)
- element 3 (the atom influencing the 1-2 bond in a bond-order sense)
- m
- c
- d
- h (cos_theta0 (can be a value -1 or 1))
- n
- beta
- lambda21, lambda2 of element 1 (1/distance units)
- lambda22, lambda2 of element 2 (1/distance units)
- B of element 1 (energy units)
- B of element 2 (energy units)
- R (cutoff, distance units, 0.5*(r_outer + r_inner))
- D (cutoff, distance units, R - r_inner)
- lambda11, lambda1 of element 1 (1/distance units)
- lambda12, lambda1 of element 2 (1/distance units)
- A of element 1 (energy units)
- A of element 2 (energy units)
- K_LP_1 (energy units, 1st order Legendre polynomial coefficient)
- K_LP_3 (energy units, 3rd order Legendre polynomial coefficient)
- K_LP_6 (energy units, 6th order Legendre polynomial coefficient)
- A123 (cos_theta, theta = equilibrium MOM or OMO bond angles)
- Aconf (cos_theta, theta = equilibrium MOM or OMO bond-bending coefficient)
- addrep (energy units, additional repulsion)
- R_omiga_a (unit-less scaler for A)
- R_omiga_b (unit-less scaler for B)
- R_omiga_c (unit-less scaler for 0.5*(lambda21+lambda22))
- R_omiga_d (unit-less scaler for 0.5*(lambda11+lambda12))
- QL1 (charge units, lower charge limit for element 1)
- QU1 (charge units, upper charge limit for element 1)
- DL1 (distance units, ion radius of element 1 with charge QL1)
- DU1 (distance units, ion radius of element 1 with charge QU1)
- QL2 (charge units, lower charge limit for element 2)
- QU2 (charge units, upper charge limit for element 2)
- DL2 (distance units, ion radius of element 2 with charge QL2)
- DU2 (distance units, ion radius of element 2 with charge QU2)
- chi (energy units, self energy 1st power term)
- dJ (energy units, self energy 2nd power term)
- dK (energy units, self energy 3rd power term)
- dL (energy units, self energy 4th power term)
- dM (energy units, self energy 6th power term)
- esm (distance units, orbital exponent)
- cmn1 (self energy penalty, rho 1 of element 1)
- cml1 (self energy penalty, rho 1 of element 2)
- cmn2 (self energy penalty, rho 2 of element 1)
- cmn2 (self energy penalty, rho 2 of element 2)
- coulcut (long range Coulombic cutoff, distance units)
- hfocor (coordination term)

The parameterization of COMB potentials start with a pure element (e.g. Si, Cu) then extend to its oxide and polymorphs (e.g. SiO2, Cu2O). For interactions not involving oxygen (e.g. Si-Cu or Hf-Zr), the COMB potential uses a mixing rule to generate these parameters. For furthur details on the parameterization and parameters, see the Tersoff doc page and the COMB publications (COMB_1) and (COMB_2). For more details on 3-body interaction types (e.g. SiSiO vs SiOSi), the mixing rule, and how to generate the potential file, please see the Tersoff doc page.

In the potentials directory, the file ffield.comb provides the LAMMPS parameters for COMB's Si, Cu, Ti, Hf and their oxides, as well as pure U, Zn and Zr metals. This file can be used for pure elements (e.g. Si, Zr), binary oxides, binary alloys (e.g. SiCu, TiZr), and complex systems. Note that alloys and complex systems require all 3-body entries be pre-defined in the potential file.

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 as described above from values in the potential file.

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

This pair style 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, pair_coeff, and fix qeq/comb commands in an input script that reads a restart file.

This pair style 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 style is part of the "manybody" package. It is only enabled if LAMMPS was built with that package (which it is by default). See the Making LAMMPS section for more info.

This pair style requires the newton setting to be "on" for pair interactions.

The COMB potentials in the ffield.comb file provided with LAMMPS (see the potentials directory) are parameterized for metal units. You can use the COMB potential with any LAMMPS units, but you would need to create your own COMB potential file with coefficients listed in the appropriate units if your simulation doesn't use "metal" units.

Related commands:

pair_style, pair_coeff, fix_qeq/comb

Default: none

(COMB_1) J. Yu, S. B. Sinnott, S. R. Phillpot, Phys Rev B, 75, 085311 (2007),

(COMB_2) T.-R. Shan, B. D. Devine, T. W. Kemper, S. B. Sinnott, S. R. Phillpot, Phys Rev B, 81, 125328 (2010).

(Tersoff) J. Tersoff, Phys Rev B, 37, 6991 (1988).

(Rick) S. W. Rick, S. J. Stuart, B. J. Berne, J Chem Phys 101, 16141 (1994).

(Wolf) D. Wolf, P. Keblinski, S. R. Phillpot, J. Eggebrecht, J Chem Phys, 110, 8254 (1999).