在in文件里，前面的unit 就是设置单位的。比如si，国际单位

http://lammps.sandia.gov/doc/units.html
A list of units. They have units that you need, for real units:
For style real, these are the units:

    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，