IMD
Central Pair Potentials
Short range pair potentials are active by default in IMD, if no other interactions have been specified with a compilation option. Pair potentials can also be explicitly switched on with the compilation option pair, for instance in combination with Tersoff or Coulomb potentials.
Pair potentials are either given in tabulated form, read from a potential file, or by analytic formulae, whose parameters are read from the parameter file. From these analytic formulae, the potential tables are then computed at the beginning of the simulation. If both a potential table and analytic potentials are specified in the parameter file, the potential table is first read in, and then the columns of those atom type pairs, for which analytic potentials are active, are overwritten. It is recommended to use only one kind of pair potential for each combination of atom types.
Tabulated Pair Potentials
The most flexible, though not necessarily the most convenient, way to specify pair potentials is by giving them in tabulated form in a potential file. The potential file containing the potential table can have two possible formats, and is specified with the parameter potfile. It is essential that the potential is tabulated equidistant in the square of the atom distance. Note that if the potential does not extend down to r^2=0, it is extrapolated linearly.
Analytically Defined Pair Potentials
Besides tabulated pair potentials read from potential files, IMD also supports some pair potentials given by analytic formulae. From these, the potential tables are then computed at the beginning of the simulation. Currently supported are Lennard-Jones potentials, Morse potentials, Buckingham potentials, and harmonic potentials to be used in the shell model together with Coulomb potentials.
All analytically defined pair potentials share common cutoff radii rcij, which have to be specified in the parameter file with the IMD parameter r_cut in the format
where N is the number of atom types. If rcij is zero for a combination ij of atom types, analytic pair potentials are not active for this combination. The potential is suitably shifted, so that it vanishes at the cutoff radius (except for the harmonic potential). In the same format as for r_cut, a starting radius for the tabulation can be specified for each pair of types, with the parameter r_begin. If this parameter is missing or zero, suitable defaults are chosen (1/10 of the nearest neighbor distance, or zero for the Morse potential and the harmonic potential). Also in the same format, the number of values in the potential table can be given for each atom type pair, with the parameter pot_res. As well as r_cut, the parameters r_begin and pot_res are commen to all analytic potentials. Except for the harmonic potential, all analytically defined potentials can be combined, even for the same atom type pair.
Lennard-Jones Potential
The Lennard-Jones potential has the following form:
The parameters epsilon and sigma have to be given in the parameter file by the IMD parameters lj_epsilon and lj_sigma in the same format as the parameter r_cut. The Lennard-Jones potential is active for any pair of atom types for which r_cut and lj_epsilon are both positive.
Lennard-Jones-Gauss Potential
The Lennard-Jones-Gauss potential has the following form:
The parameters epsilon and sigma have to be given in the parameter file by the IMD parameters lj_epsilon and lj_sigma as before for the pure Lennard-Jones potential in the same format as the parameter r_cut. The additional parameters epsilon_G, r0_G and sigma_G have to be given in the parameter file by the IMD parameters ljg_eps, ljg_r0 and ljg_sig.
Morse Potential
The Morse potential has the following form:
The parameters epsilon, sigma, and alpha have to be given in the parameter file with the IMD parameters morse_epsilon, morse_sigma, and morse_alpha in the same format as the paramter r_cut. The Morse potential is active for any pair of atom types for which r_cut and morse_epsilon are both positive.
Morse-stretch Potential
The Morse-stretch potential has the following form:
The parameters ms_D, ms_gamma and ms_r0 are the standard parameters of the Morse-stretch potential. A harmonic term is added to the potential if r < ms_rmin to avoid unphysical crashes, ms_harm_C is the spring constant for the harmonic term.
Buckingham Potential
The Buckingham potential has the following form:
The parameters A, C, and sigma have to be given in the parameter file with the IMD parameters buck_a, buck_c, and buck_sigma in the same format as the paramter r_cut. The Buckingham potential is active for any pair of atom types for which r_cut and buck_sigma are both positive.
Harmonic Potential
To be used in the shell model, a harmonic potential between the ion core and its valence shell can be defined. The nearest neighbor distance is zero, so that only the spring constant has to be specified with the parameter spring_const. As only shells and cores interact with this potential, only the spring constants for atom type pairs ij with i different from j have to be specified. For i=j, the spring constant is assumed to be zero. The harmonic potential is active for any pair of atom types for which r_cut and spring_const are both positive. It then replaces any other pair potential that might be active.