lammps学习L-J势函数涉及各物理量单位体制

1、LJ势函数单位体制The lj/cut styles compute the standard 12/6 Lennard-Jones potential, given by Rc is the cutoff.units ljFor style lj, all quantities are unitless. Without loss of generality, LAMMPS sets the fundamental quantities mass, sigma, epsilon, and the Boltzmann constant = 1. The masses, distances, e

2、nergies you specify are multiples of these fundamental values. The formulas relating the reduced or unitless quantity (with an asterisk) to the same quantity with units is also given. Thus you can use the mass & sigma & epsilon values for a specific material and convert the results from a unitless L

3、J simulation into physical quantities.mass = mass or m distance = sigma, where x* = x / sigma time = tau, where t* = t (epsilon / m / sigma2)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）= ep

4、silon, where t* = t / epsilon temperature = reduced LJ temperature, where T* = T Kb / epsilon pressure = reduced LJ pressure, where P* = P sigma3 / epsilon dynamic viscosity = reduced LJ viscosity, where eta* = eta sigma3 / epsilon / tau charge = reduced LJ charge, where q* = q / (4 pi perm0 sigma e

5、psilon)1/2 dipole = reduced LJ dipole, moment where *mu = mu / (4 pi perm0 sigma3 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 sigmadimLamps中的LJ单位问题LJ单位mass：，即1mass相当于sigma：，即1sigma相当于epsilon：，即1epsilon相

6、当于那么根据manual里面的dipole的解释，即：dipole = reduced LJ dipole, moment where *mu = mu / (4 pi perm0 sigma3 epsilon)1/2所以1dipole相当于 (4 pi perm0 sigma3 epsilon)1/2即：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，也只有这样，dipole的量纲分析才正确。那么：lattice commandSyntax:lattice style scale keyword values . styl

7、e =noneorscorbccorfccorhcpordiamondorsqorsq2orhexorcustom scale = scale factor between lattice and simulation boxscale = 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 un

8、its defined by the unit style. For example, inrealormetalunits, if the unit cell is a unit cube with edge length 1.0, specifying scale = 3.52 would 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-J

9、ones reduced density, typically written as rho*. LAMMPS converts this value into the multiplicative factor via the formula factordim = 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

10、 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factordim = 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

11、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_styleDefault:thermo_style oneStyle one prints a one-line summary of thermodynamic info that is the equivale

12、nt of thermo_style custom step 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 custom step temp epair emol etotal presscustom:stylestep = timest

13、eptemp = temperatureepair = pairwise energy (evdwl + ecoul + elong + etail)emol = molecular energy (ebond + eangle + edihed + eimp)etotal = total energy (pe + ke)press = pressurevol = volumethermo_style custom step temp pe etotal ke press pxx pyy pzz vol #c_1x c_1y c_1zpe = total potential energyke

14、= kinetic energypxx,pyy,pzz,pxy,pxz,pyz = 6 components of pressure tensorthermo_modifySyntax:thermo_modify keyword valuetemp value = compute ID that calculates a temperaturedump#dump1 all atom 500 dump.friction.lammpstrj#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.frictionFor style atom, 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 o