lammps学习L-J势函数涉及各物理量单位体制

LJ 势函数单位体制

Thelj/cutstyles compute the standard 12/6 Lennard-Jones potential, given by

1264[()()]E r r

σσε=-c r r <

Rc is the cutoff.

units lj

For stylelj, all quantities are unitless. Without loss of generality, LAMMPS sets the fundamental quantitiesmass, sigma, epsilon, and the Boltzmann constant = 1. The masses, distances, energies you specify aremultiples of these fundamental values. The formulas relating the reduced or unitless quantity (with anasterisk) to the same quantity with units is also given. Thus you can use the mass & sigma & epsilon valuesfor a specific material and convert the results from a unitless LJ simulation into physical quantities.

mass = mass or m ?

distance = sigma, where x* = x / sigma ?

time = tau, where t* = t (epsilon / m / sigma^2)^1/2 ?

energy = epsilon, where E* = E / epsilon ?

velocity = sigma/tau, where v* = v tau / sigma ?

force = epsilon/sigma, where f* = f sigma / epsilon ?

torque （力矩）= epsilon, where t* = t / epsilon ?

temperature = reduced LJ temperature, where T* = T Kb / epsilon ? pressure = reduced LJ pressure, where P* = P sigma^3 / epsilon ?

?dynamic viscosity = reduced LJ viscosity, where eta* = eta sigma^3 / epsilon / tau

? charge = reduced LJ charge, where q* = q / (4 pi perm0 sigma epsilon)^1/2

? dipole = reduced LJ dipole, moment where *mu = mu / (4 pi perm0 sigma^3 epsilon)^1/2

? electric field = force/charge, where E* = E (4 pi perm0 sigma epsilon)^1/2 sigma / epsilon

? density = mass/volume, where rho* = rho sigma^dim

Lamps 中的LJ 单位问题

LJ 单位

mass ：82.4310kg -=?，即1mass 相当于82.4310kg -?

sigma ：5110m -=?，即1sigma 相当于5110m -?

epsilon ：138.4110N m -=??，即1epsilon 相当于138.4110N m -??

那么根据manual 里面的dipole 的解释，即：

dipole = reduced LJ dipole, moment where *mu = mu / (4 pi perm0 sigma^3 epsilon)^1/2

所以1dipole 相当于 (4 pi perm0 sigma^3 epsilon)^1/2

根据您的解释，这里的perm0应该也是一个约化单位，即perm0=1，但在计算的过程中要转化成实际单位，故应该此处的721

0410N/A perm π-=?，也只有这样，dipole 的量纲分析才正确。那么：

1129.1710A m -===??

lattice command

Syntax:

lattice style scale keyword values ...

? style =noneorscorbccorfccorhcpordiamondorsqorsq2orhexorcustom ? scale = scale factor between lattice and simulation box

scale = reduced density rho* (for LJ units)

scale = lattice constant in distance units (for all other units)

For all unit styles exceptlj, the scale argument is specified in the distance units defined by the unit style. Forexample, inrealormetalunits, if the unit cell is a unit cube with edge length 1.0, specifying scale =

3.52would create a cubic lattice with a spacing of 3.52 Angstroms. Incgsunits, the spacing would be 3.52 cm.

For unit style lj, the scale argument is the Lennard-Jones reduced

density, typically written as rho*. LAMMPS converts this value into the multiplicative factor via the formula "factor^dim = rho/rho*", where rho = N/V with V = the volume of the lattice unit cell and N = the number of basis atoms in the unit cell (described below), and dim = 2 or 3 for the dimensionality of the simulation. Effectively, this means that if LJ particles of size sigma = 1.0 are used in the simulation, the lattice of particles will be at the desired reduced density.

对于lj单元，scale是Lennard-Jones折算密度，典型写为rho*。Lammps通过公式"factor^dim = rho/rho*"将这个值转变为倍增因子。Rho=N/V，v=晶格单元的体积，n=晶胞中基本原子的数量，dim根据模拟的维度等于2或者3。实际上这表明如果在模拟中使用的lj粒子的sigma=1.0，那么晶格中的粒子将会有预期的折算密度。

Thermo输出量

The content and format of what is printed is controlled by the thermo_style and thermo_modify commands.

thermo_style

Default:

thermo_style one

Styleoneprints a one-line summary of thermodynamic info that is the equivalent of "thermo_style customstep temp epair emol etotal press". The line contains only numeric values.

All styles exceptcustomhavevolappended to their list of outputs if

the simulation box volume changes.

during the simulation.

thermo_style customstep temp epair emol etotal press

custom:style

step = timestep

temp = temperature

epair = pairwise energy (evdwl + ecoul + elong + etail)

emol = molecular energy (ebond + eangle + edihed + eimp)

etotal = total energy (pe + ke)

press = pressure

vol = volume

thermo_style custom step temp pe etotal ke press pxx pyy pzz vol #c_1x c_1y c_1z

pe = total potential energy

ke = kinetic energy

pxx,pyy,pzz,pxy,pxz,pyz = 6 components of pressure tensor thermo_modify

Syntax:

thermo_modify keyword value

tempvalue = compute ID that calculates a temperature

dump

#dump 1 all atom 500 https://www.360docs.net/doc/c06276985.html,mmpstrj

#1（dump ID）all（group ID）,atom（style）每隔500步将原子信息（atom ID、atom type、atom coordinates）写入文件dump.friction For styleatom, atom coordinates are written to the file, along with the atom ID and atom type. By default,atom coords are written in a scaled format (from 0 to 1). I.e. an x value of 0.25 means the atom is at a location1/4 of the distance from xlo to xhi of the box boundaries. The format can be changed to unscaled coords viathe dump_modifysettings. Image flags can also be added for each atom via dump_modify.