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1、LJ 势函数单位体制The lj/cut styles compute the standard 12/6 Lennard-Jones potential, given by1264 ()() ErrcrrRc 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 mass
2、es, distances, energies 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 f
3、rom a unitless LJ 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
4、 torque （力矩）= epsilon, 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
5、pi perm0 sigma epsilon)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82.43 10 kg82.
6、43 10 kgsigma：，即 1sigma 相当于51 10 m 51 10 mepsilon：，即 1epsilon 相当于138.41 10N m138.41 10N 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 sigma3 epsilon)1/2所以 1dipole 相当于 (4 pi perm0 sigma3 epsilon)1/2即：531340 (1 10 m)8.41 10N mperm根据您的解释，这里的 perm0 应该也是一个约
7、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：7210410N/Aperm53135313 722111240 (1 10 m)8.41 10N m14(1 10 m)8.41 10N m410 N/A8.41 109.17 10A mperm lattice commandSyntax:lattice style scale keyword values . style =noneorscorbccorfccorhcpordiamondorsqorsq2orhexorcustom scale = scale fac
8、tor 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 units defined by the unit style. For example, inrealormetalunits, if the unit
9、 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-Jones reduced density, typically written as rho*. LAMMPS converts this value
10、 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 dimensionality of the simulation. Effectively, this means that if LJ par
11、ticles 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
12、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 equivalent of “thermo_style custom step temp epair emol etotal pre
13、ss“. 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 = timesteptemp = temperatureepair = pairwise energy (evdwl + eco
14、ul + 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 = kinetic energypxx,pyy,pzz,pxy,pxz,pyz = 6 components o
15、f 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
16、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_modify settings. Image flags can also be added for each atom via dump_modify.
