# pair_style buck/long/coul/long command Accelerator Variants: *buck/long/coul/long/omp* ## Syntax ``` LAMMPS pair_style buck/long/coul/long flag_buck flag_coul cutoff (cutoff2) ``` - flag_buck = *long* or *cut* *long* = use Kspace long-range summation for the dispersion term 1/r\^6 *cut* = use a cutoff - flag_coul = *long* or *off* *long* = use Kspace long-range summation for the Coulombic term 1/r *off* = omit the Coulombic term - cutoff = global cutoff for Buckingham (and Coulombic if only 1 cutoff) (distance units) - cutoff2 = global cutoff for Coulombic (optional) (distance units) ## Examples ``` LAMMPS pair_style buck/long/coul/long cut off 2.5 pair_style buck/long/coul/long cut long 2.5 4.0 pair_style buck/long/coul/long long long 4.0 pair_coeff * * 1 1 pair_coeff 1 1 1 3 4 ``` ## Description The *buck/long/coul/long* style computes a Buckingham potential (exp/6 instead of Lennard-Jones 12/6) and Coulombic potential, given by $$\begin{aligned} E = & A e^{-r / \rho} - \frac{C}{r^6} \qquad r < r_c \\ E = & \frac{C q_i q_j}{\epsilon r} \qquad r < r_c \end{aligned}$$ $r_c$ is the cutoff. If one cutoff is specified in the pair_style command, it is used for both the Buckingham and Coulombic terms. If two cutoffs are specified, they are used as cutoffs for the Buckingham and Coulombic terms respectively. The purpose of this pair style is to capture long-range interactions resulting from both attractive 1/r\^6 Buckingham and Coulombic 1/r interactions. This is done by use of the *flag_buck* and *flag_coul* settings. The [Ismail](Ismail) paper has more details on when it is appropriate to include long-range 1/r\^6 interactions, using this potential. If *flag_buck* is set to *long*, no cutoff is used on the Buckingham 1/r\^6 dispersion term. The long-range portion can be calculated by using the [kspace_style ewald/disp or pppm/disp](kspace_style) commands. The specified Buckingham cutoff then determines which portion of the Buckingham interactions are computed directly by the pair potential versus which part is computed in reciprocal space via the Kspace style. If *flag_buck* is set to *cut*, the Buckingham interactions are simply cutoff, as with [pair_style buck](pair_buck). If *flag_coul* is set to *long*, no cutoff is used on the Coulombic interactions. The long-range portion can calculated by using any of several [kspace_style](kspace_style) command options such as *pppm* or *ewald*. Note that if *flag_buck* is also set to long, then the *ewald/disp* or *pppm/disp* Kspace style needs to be used to perform the long-range calculations for both the Buckingham and Coulombic interactions. If *flag_coul* is set to *off*, Coulombic interactions are not computed. The following coefficients must be defined for each pair of atoms types via the [pair_coeff](pair_coeff) command as in the examples above, or in the data file or restart files read by the [read_data](read_data) or [read_restart](read_restart) commands: - A (energy units) - rho (distance units) - C (energy-distance\^6 units) - cutoff (distance units) - cutoff2 (distance units) The second coefficient, rho, must be greater than zero. The latter 2 coefficients are optional. If not specified, the global Buckingham and Coulombic cutoffs specified in the pair_style command are used. If only one cutoff is specified, it is used as the cutoff for both Buckingham and Coulombic interactions for this type pair. If both coefficients are specified, they are used as the Buckingham and Coulombic cutoffs for this type pair. Note that if you are using *flag_buck* set to *long*, you cannot specify a Buckingham cutoff for an atom type pair, since only one global Buckingham cutoff is allowed. Similarly, if you are using *flag_coul* set to *long*, you cannot specify a Coulombic cutoff for an atom type pair, since only one global Coulombic cutoff is allowed. ------------------------------------------------------------------------ Styles with a *gpu*, *intel*, *kk*, *omp*, or *opt* suffix are functionally the same as the corresponding style without the suffix. They have been optimized to run faster, depending on your available hardware, as discussed on the [Accelerator packages](Speed_packages) page. The accelerated styles take the same arguments and should produce the same results, except for round-off and precision issues. These accelerated styles are part of the GPU, INTEL, KOKKOS, OPENMP, and OPT packages, respectively. They are only enabled if LAMMPS was built with those packages. See the [Build package](Build_package) page for more info. You can specify the accelerated styles explicitly in your input script by including their suffix, or you can use the [-suffix command-line switch](Run_options) when you invoke LAMMPS, or you can use the [suffix](suffix) command in your input script. See the [Accelerator packages](Speed_packages) page for more instructions on how to use the accelerated styles effectively. ------------------------------------------------------------------------ ## Mixing, shift, table, tail correction, restart, rRESPA info This pair style does not support mixing. Thus, coefficients for all I,J pairs must be specified explicitly. This pair style supports the [pair_modify](pair_modify) shift option for the energy of the exp() and 1/r\^6 portion of the pair interaction, assuming *flag_buck* is *cut*. This pair style does not support the [pair_modify](pair_modify) shift option for the energy of the Buckingham portion of the pair interaction. This pair style supports the [pair_modify](pair_modify) table and table/disp options since they can tabulate the short-range portion of the long-range Coulombic and dispersion interactions. This pair style write its information to [binary restart files](restart), so pair_style and pair_coeff commands do not need to be specified in an input script that reads a restart file. This pair style supports the use of the *inner*, *middle*, and *outer* keywords of the [run_style respa](run_style) command, meaning the pairwise forces can be partitioned by distance at different levels of the rRESPA hierarchy. See the [run_style](run_style) command for details. ------------------------------------------------------------------------ ## Restrictions This style is part of the KSPACE package. It is only enabled if LAMMPS was built with that package. See the [Build package](Build_package) page for more info. ## Related commands [pair_coeff](pair_coeff) ## Default none ------------------------------------------------------------------------ ::: {#Ismail} **(Ismail)** Ismail, Tsige, In \'t Veld, Grest, Molecular Physics (accepted) (2007). :::