# dihedral_style class2 command Accelerator Variants: *class2/omp*, *class2/kk* ## Syntax ``` LAMMPS dihedral_style class2 ``` ## Examples ``` LAMMPS dihedral_style class2 dihedral_coeff 1 100 75 100 70 80 60 dihedral_coeff * mbt 3.5945 0.1704 -0.5490 1.5228 dihedral_coeff * ebt 0.3417 0.3264 -0.9036 0.1368 0.0 -0.8080 1.0119 1.1010 dihedral_coeff 2 at 0.0 -0.1850 -0.7963 -2.0220 0.0 -0.3991 110.2453 105.1270 dihedral_coeff * aat -13.5271 110.2453 105.1270 dihedral_coeff * bb13 0.0 1.0119 1.1010 ``` ## Description The *class2* dihedral style uses the potential $$\begin{aligned} E = & E_d + E_{mbt} + E_{ebt} + E_{at} + E_{aat} + E_{bb13} \\ E_d = & \sum_{n=1}^{3} K_n [ 1 - \cos (n \phi - \phi_n) ] \\ E_{mbt} = & (r_{jk} - r_2) [ A_1 \cos (\phi) + A_2 \cos (2\phi) + A_3 \cos (3\phi) ] \\ E_{ebt} = & (r_{ij} - r_1) [ B_1 \cos (\phi) + B_2 \cos (2\phi) + B_3 \cos (3\phi) ] + \\ & (r_{kl} - r_3) [ C_1 \cos (\phi) + C_2 \cos (2\phi) + C_3 \cos (3\phi) ] \\ E_{at} = & (\theta_{ijk} - \theta_1) [ D_1 \cos (\phi) + D_2 \cos (2\phi) + D_3 \cos (3\phi) ] + \\ & (\theta_{jkl} - \theta_2) [ E_1 \cos (\phi) + E_2 \cos (2\phi) + E_3 \cos (3\phi) ] \\ E_{aat} = & M (\theta_{ijk} - \theta_1) (\theta_{jkl} - \theta_2) \cos (\phi) \\ E_{bb13} = & N (r_{ij} - r_1) (r_{kl} - r_3) \end{aligned}$$ where $E_d$ is the dihedral term, $E_{mbt}$ is a middle-bond-torsion term, $E_{ebt}$ is an end-bond-torsion term, $E_{at}$ is an angle-torsion term, $E_{aat}$ is an angle-angle-torsion term, and $E_{bb13}$ is a bond-bond-13 term. $\theta_1$ and $\theta_2$ are equilibrium angles and $r_1$, $r_2$, and $r_3$ are equilibrium bond lengths. See [(Sun)](dihedral-Sun) for a description of the COMPASS class2 force field. Coefficients for the $E_d$, $E_{mbt}$, $E_{ebt}$, $E_{at}$, $E_{aat}$, and $E_{bb13}$ formulas must be defined for each dihedral type via the [dihedral_coeff](dihedral_coeff) command as in the example above, or in the data file or restart files read by the [read_data](read_data) or [read_restart](read_restart) commands. These are the 6 coefficients for the $E_d$ formula: - $K_1$ (energy) - $\phi_1$ (degrees) - $K_2$ (energy) - $\phi_2$ (degrees) - $K_3$ (energy) - $phi_3$ (degrees) For the $E_{mbt}$ formula, each line in a [dihedral_coeff](dihedral_coeff) command in the input script lists 5 coefficients, the first of which is *mbt* to indicate they are MiddleBondTorsion coefficients. In a data file, these coefficients should be listed under a *MiddleBondTorsion Coeffs* heading and you must leave out the *mbt*, i.e. only list 4 coefficients after the dihedral type. - *mbt* - $A_1$ (energy/distance) - $A_2$ (energy/distance) - $A_3$ (energy/distance) - $r_2$ (distance) For the $E_{ebt}$ formula, each line in a [dihedral_coeff](dihedral_coeff) command in the input script lists 9 coefficients, the first of which is *ebt* to indicate they are EndBondTorsion coefficients. In a data file, these coefficients should be listed under a *EndBondTorsion Coeffs* heading and you must leave out the *ebt*, i.e. only list 8 coefficients after the dihedral type. - *ebt* - $B_1$ (energy/distance) - $B_2$ (energy/distance) - $B_3$ (energy/distance) - $C_1$ (energy/distance) - $C_2$ (energy/distance) - $C_3$ (energy/distance) - $r_1$ (distance) - $r_3$ (distance) For the $E_{at}$ formula, each line in a [dihedral_coeff](dihedral_coeff) command in the input script lists 9 coefficients, the first of which is *at* to indicate they are AngleTorsion coefficients. In a data file, these coefficients should be listed under a *AngleTorsion Coeffs* heading and you must leave out the *at*, i.e. only list 8 coefficients after the dihedral type. - *at* - $D_1$ (energy) - $D_2$ (energy) - $D_3$ (energy) - $E_1$ (energy) - $E_2$ (energy) - $E_3$ (energy) - $\theta_1$ (degrees) - $\theta_2$ (degrees) $\theta_1$ and $\theta_2$ are specified in degrees, but LAMMPS converts them to radians internally; hence the various $D$ and $E$ are effectively energy per radian. For the $E_{aat}$ formula, each line in a [dihedral_coeff](dihedral_coeff) command in the input script lists 4 coefficients, the first of which is *aat* to indicate they are AngleAngleTorsion coefficients. In a data file, these coefficients should be listed under a *AngleAngleTorsion Coeffs* heading and you must leave out the *aat*, i.e. only list 3 coefficients after the dihedral type. - *aat* - $M$ (energy) - $\theta_1$ (degrees) - $\theta_2$ (degrees) $\theta_1$ and $\theta_2$ are specified in degrees, but LAMMPS converts them to radians internally; hence $M$ is effectively energy per radian\^2. For the $E_{bb13}$ formula, each line in a [dihedral_coeff](dihedral_coeff) command in the input script lists 4 coefficients, the first of which is *bb13* to indicate they are BondBond13 coefficients. In a data file, these coefficients should be listed under a *BondBond13 Coeffs* heading and you must leave out the *bb13*, i.e. only list 3 coefficients after the dihedral type. - *bb13* - $N$ (energy/distance\^2) - $r_1$ (distance) - $r_3$ (distance) ------------------------------------------------------------------------ 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. ------------------------------------------------------------------------ ## Restrictions This dihedral style can only be used if LAMMPS was built with the CLASS2 package. See the [Build package](Build_package) doc page for more info. ## Related commands [dihedral_coeff](dihedral_coeff) ## Default none ------------------------------------------------------------------------ ::: {#dihedral-Sun} **(Sun)** Sun, J Phys Chem B 102, 7338-7364 (1998). :::