forked from lijiext/lammps
94 lines
2.9 KiB
Plaintext
94 lines
2.9 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
|
|
|
|
:link(lws,http://lammps.sandia.gov)
|
|
:link(ld,Manual.html)
|
|
:link(lc,Section_commands.html#comm)
|
|
|
|
:line
|
|
|
|
improper_style class2 command :h3
|
|
|
|
[Syntax:]
|
|
|
|
improper_style class2 :pre
|
|
|
|
[Examples:]
|
|
|
|
improper_style class2
|
|
improper_coeff 1 100.0 0 :pre
|
|
|
|
[Description:]
|
|
|
|
The {class2} improper style uses the potential
|
|
|
|
:c,image(Eqs/improper_class2.jpg)
|
|
|
|
where Ei is the improper term and Eaa is an angle-angle term. The 3 X
|
|
terms in Ei are an average over 3 out-of-plane angles.
|
|
|
|
The 4 atoms in an improper quadruplet (listed in the data file read by
|
|
the "read_data"_read_data.html command) are ordered I,J,K,L. X_IJKL
|
|
refers to the angle between the plane of I,J,K and the plane of J,K,L,
|
|
and the bond JK lies in both planes. Similarly for X_KJLI and X_LJIK.
|
|
Note that atom J appears in the common bonds (JI, JK, JL) of all 3 X
|
|
terms. Thus J (the 2nd atom in the quadruplet) is the atom of
|
|
symmetry in the 3 X angles.
|
|
|
|
The subscripts on the various theta's refer to different combinations
|
|
of 3 atoms (I,J,K,L) used to form a particular angle. E.g. Theta_IJL
|
|
is the angle formed by atoms I,J,L with J in the middle. Theta1,
|
|
theta2, theta3 are the equilibrium positions of those angles. Again,
|
|
atom J (the 2nd atom in the quadruplet) is the atom of symmetry in the
|
|
theta angles, since it is always the center atom.
|
|
|
|
Since atom J is the atom of symmetry, normally the bonds J-I, J-K, J-L
|
|
would exist for an improper to be defined between the 4 atoms, but
|
|
this is not required.
|
|
|
|
See "(Sun)"_#Sun for a description of the COMPASS class2 force field.
|
|
|
|
The following coefficients must be defined for each improper type via the
|
|
"improper_coeff"_improper_coeff.html command as in the example above, or in
|
|
the data file or restart files read by the "read_data"_read_data.html
|
|
or "read_restart"_read_restart.html commands:
|
|
|
|
For this style, coefficients for the Ei formula can be specified in
|
|
either the input script or data file. These are the 2 coefficients:
|
|
|
|
K (energy/radian^2)
|
|
X0 (degrees) :ul
|
|
|
|
X0 is specified in degrees, but LAMMPS converts it to radians
|
|
internally; hence the units of K are in energy/radian^2.
|
|
|
|
Coefficients for the Eaa formula can only be specified in the data
|
|
file. For the Eaa formula, the coefficients are listed under a
|
|
"AngleAngle Coeffs" heading and each line lists 6 coefficients:
|
|
|
|
M1 (energy/distance)
|
|
M2 (energy/distance)
|
|
M3 (energy/distance)
|
|
theta1 (degrees)
|
|
theta2 (degrees)
|
|
theta3 (degrees) :ul
|
|
|
|
The theta values are specified in degrees, but LAMMPS converts them to
|
|
radians internally; hence the units of M are in energy/radian^2.
|
|
|
|
[Restrictions:]
|
|
|
|
This improper style can only be used if LAMMPS was built with the
|
|
"class2" package. See the "Making LAMMPS"_Section_start.html#2_3
|
|
section for more info on packages.
|
|
|
|
[Related commands:]
|
|
|
|
"improper_coeff"_improper_coeff.html
|
|
|
|
[Default:] none
|
|
|
|
:line
|
|
|
|
:link(Sun)
|
|
[(Sun)] Sun, J Phys Chem B 102, 7338-7364 (1998).
|