LAMMPS

文章来源：“分子动力学”公众号

链接：https://mp.weixin.qq.com/s/4BIOPWJw-pCjCw_v9GGqDA

units命令用来定义模拟过程中使用的单位类型，它决定了所有输入脚本、数据文件和所有输出到屏幕、日志文件以及dump文件中物理量的单位。

使用语法

units style style可取：lj/real/metal/si/cgs/electron

使用举例

units metal units lj

使用介绍

该命令用来设置模拟中使用的单位类型。它决定了所有输入脚本、数据文件和所有输出到屏幕、日志文件以及dump文件中物理量的单位。一般来说，该命令用在输入脚本最开始的位置。

除了lj类型之外，其他所有的单位类型所用到的物理常量都来自网站：http://www.physics.nist.gov。real类型中Kcal的定义，LAMMPS使用热力学卡=4.184J。

LJ类型

对于lj类型，所有的物理量都是没有单位的。不失一般性，LAMMPS将基本量mass、sigma、epsilon和波尔兹曼常数设为1。你所指定的质量、距离、能量就是这些基本量的倍数。下面给出了无单位量（用*标出）与有单位量之间的换算关系。基于此，你可以将lj模拟无单位量转换为正常的物理量。

mass = mass or m distance = sigma, where x* = x / sigma time = tau, where tau = 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 = 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 需要注意的是，lj单位类型下，通过命令thermo_style设置的热力学信息的输出是将能量对原子数量进行了归一化，即能量/原子。这可以通过命令thermo_modify norm进行修改。

real类型

mass = grams/mole distance = Angstroms time = femtoseconds energy = Kcal/mole velocity = Angstroms/femtosecond force = Kcal/mole-Angstrom torque = Kcal/mole temperature = Kelvin pressure = atmospheres dynamic viscosity = Poise charge = multiple of electron charge (+1.0 is a proton) dipole = charge*Angstroms electric field = volts/Angstrom density = gram/cm^dim

metal类型

mass = grams/mole distance = Angstroms time = picoseconds energy = eV velocity = Angstroms/picosecond force = eV/Angstrom torque = eV temperature = Kelvin pressure = bars dynamic viscosity = Poise charge = multiple of electron charge (+1.0 is a proton) dipole = charge*Angstroms electric field = volts/Angstrom density = gram/cm^dim

si类型

mass = kilograms distance = meters time = seconds energy = Joules velocity = meters/second force = Newtons torque = Newton-meters temperature = Kelvin pressure = Pascals dynamic viscosity = Pascalsecond charge = Coulombs dipole = Coulombsmeters electric field = volts/meter density = kilograms/meter^dim

cgs类型

mass = grams distance = centimeters time = seconds energy = ergs velocity = centimeters/second force = dynes torque = dyne-centimeters temperature = Kelvin pressure = dyne/cm^2 or barye = 1.0e-6 bars dynamic viscosity = Poise charge = statcoulombs or esu dipole = statcoul-cm = 10^18 debye electric field = statvolt/cm or dyne/esu density = grams/cm^dim

electron类型

mass = atomic mass units distance = Bohr time = femtoseconds energy = Hartrees velocity = Bohr/atomic time units [1.03275e-15 seconds] force = Hartrees/Bohr temperature = Kelvin pressure = Pascals charge = multiple of electron charge (+1.0 is a proton) dipole moment = Debye electric field = volts/cm

不同的单位类型，也设置了默认的时间步和领域表层距离，如下：

For style lj these are dt = 0.005 tau and skin = 0.3 sigma. For style real these are dt = 1.0 fmsec and skin = 2.0 Angstroms. For style metal these are dt = 0.001 psec and skin = 2.0 Angstroms. For style si these are dt = 1.0e-8 sec and skin = 0.001 meters. For style cgs these are dt = 1.0e-8 sec and skin = 0.1 cm. For style electron these are dt = 0.001 fmsec and skin = 2.0 Bohr.

使用限制 该命令不能在模拟盒子被定义（使用命令read_data或create_box）之后使用。

默认设置 units lj