unit converter

Conventional units:

• distance = Angstroms
• time = femtoseconds
• mass = grams/mole
• temperature = degrees K
• pressure = atmospheres
• energy = Kcal/mole
• velocity = Angstroms/femtosecond
• force = grams/mole * Angstroms/femtosecond^2
• charge = +/- 1.0 is proton/electron

LJ reduced units:

• distance = sigmas
• time = reduced LJ tau
• mass = ratio to unitless 1.0
• temperature = reduced LJ temp
• pressure = reduced LJ pressure
• energy = epsilons
• velocity = sigmas/tau
• force = reduced LJ force (sigmas/tau^2)
• charge = ratio to unitless 1.0

This listing of variables assumes conventional units; to convert to LJ reduced units, simply substitute the appropriate term from the list above. E.g. x is in sigmas in LJ units. Per-mole in any of the units simply means for 6.023 x 10^23 atoms.

Meaning        Variable        Units

positions      x               Angstroms
velocities     v               Angstroms / click (see below)
forces         f               Kcal / (mole - Angstrom)                
masses         mass            gram / mole
charges        q               electron units (-1 for an electron)
                                 (1 e.u. = 1.602 x 10^-19 coul)

time            ---             clicks (1 click = 48.88821 fmsec)
timestep        dt              clicks
input timestep  dt_in           fmsec
time convert    dtfactor        48.88821 fmsec / click

temperature     t_current       degrees K
                t_start
                t_stop
input damping   t_freq_in       inverse fmsec
internal temp   t_freq          inverse clicks
  damping

dielec const    dielectric      1.0 (unitless)
Boltmann const  boltz           0.001987191 Kcal / (mole - degree K)

virial          virial[xyz]     Kcal/mole = r dot F
pressure factor pfactor         68589.796 (convert internal to atmospheres)
internal        p_current       Kcal / (mole - Angs^3)
  pressure      p_start
                p_stop
input press     p_start_in      atmospheres
                p_stop_in
output press    log file        atmospheres
input damping   p_freq_in       inverse time
internal press  p_freq          inverse clicks
  damping

pot eng         e_potential     Kcal/mole
kin eng         e_kinetic       Kcal/mole
eng convert     efactor         332.0636 (Kcal - Ang) / (q^2 - mole)
                                (convert Coulomb eng to Kcal/mole)

LJ coeffs       lja,ljb         Kcal-Angs^(6,12)/mole

bond            various         see force_fields file
  parameters    2,3,4-body
                terms

參考
https://lammps.sandia.gov/doc/99/units.html

https://www.unitconverters.net/

Time
1 fs = 10^-15 s
1 ps = 10^-12 s

Pressure
1 pascal (Pa) = 1 newton/square meter (N/m2)
1 bar = 0.98692 atmosphere (atm) = 10^5 pascals (Pa)
1 atm = 101325 pascals (Pa)
1 pascals (Pa) = 0.0000098692 atm
1 GPa =10^9 Pa
1 MPa =10^6 Pa

Energy
1 joule (J) = 1 newton meter (Nm)
1 calorie (cal) = 4.184 joule (J)

Gas constant

pV = nRT
R is related to the Boltzmann constant, k, by

R = k NA
where k = 1.3806 x 10^-23 J K^-1, and NA = 6.022 x 10^23 mol^-1

R with different units 

8.31451 J K^-1 mol^-1 

8.20578 x 10^-2 L atm K^-1 mol^-1 

8.31451 x 10^-2 L bar K^-1 mol^-1 

8.31451 Pa m^3 K^-1 mol^-1 

1.98722 cal K^-1 mol^-1

1 Pa = 10^-3/4.184 kcal/m^3 = 10^-33/4.184 kcal/A^3 = 6.02*10^23/4.184/10^33 kcal/mol/A^3
1 GPa = 10^6 Pa = 6.02*10^29/4.184/10^33 kcal/mol/A^3 = 10^-4*6.02/4.184 kcal/mol/A^3

1 kcal/mol/A^3 = 4184*10^30/6/10^23 j/m^3 (Pa) * 0.0000098692 (atm/Pa) = 412927.328 atm

timestep (fs轉無因次)
t* = t * (epsilon / m / sigma^2)^1/2
t* = t([fs]) * [ sqrt( Kcal/mol * mol/g * Angstrom^-2 ) ] (1 Kcal = 4184J)
    = t([fs]) * [ sqrt( 4184*Kg*meter^2/(second^2) * 1/g *  1/(10^-20 meter^2) ) ] (1 Angstrom = 10^-10 meter)
    = t([fs]) * [ sqrt( 4184*1000 g*meter/(10^30 fs^2) * 1/g *  1/(10^-20 meter^2) ) ] (1 second = 10^15 femtosecond)
    = t * 0.020455

pressure (atm轉無因次)
P* = P([atm]) sigma^3 / epsilon (1 atm = 101325 Pa = 101325 Kg/m/s^2)
     = P([101325 Kg/meter/s^2]) * [ Angstrom^-3 * mol/Kcal ] (1 Kcal = 4184J)
     = P([101325 Kg/meter/s^2]) * [10^-30 meter * mol*s^2/4184/Kg/meter] (1 mol = 6*10^23)
     = P([101325 Kg/meter/s^2]) * [10^-30 meter * 6*10^23*s^2/4184/Kg/meter]
     = P * 0.00001453

temperature (K轉無因次)
T* = T Kb / epsilon
     = T([K]) * [ 1.38064852*10^-23 J/K * mol/Kcal ] (1 mol = 6*10^23)
     = T([K]) * [ 1.38064852*10^-23 J/K * 6*10^23/4184 J ]
     = T * 0.0019798975