Simbody  3.6
Preprocessor Macro Constants

Modules

 Mathematical Constants
 
 Physical Constants
 
 Unit Conversion Factors
 

Detailed Description

These are preprocessor (#define) macros providing constants values at very high precision. See the discussion under the main Predefined Constants module heading.

Units

Our most common unit systems are the "SI" (MKS) system, and the "MD" system used for molecular dynamics. SI units are meters, kg, seconds, coulombs (ampere-s), kelvins and moles. MD units are nanometers, atomic mass units (Daltons, g/mol), picoseconds, proton charge e, kelvins, and moles. Many molecular dynamicists and chemists prefer kcals for energy and angstroms for length. This does not constitute a consistent set of units, however, so we provide for it by conversion from the MD units, which are consistent. (By consistent, we mean that force units = mass-length/time^2, so f=ma!)

Unit systems
           SI (MKS)           MD                     KCAL-ANGSTROM
---------  --------------  ------------------------  ------------------
length     meter           nanometer                 angstrom (A)
mass       kg              amu, dalton               amu, dalton
time       second          picosecond                picosecond
charge     coulomb         e, proton charge          e, proton charge
temp.      kelvin          kelvin                    kelvin
substance  mole            mole                      mole
velocity   m/s             km/s (nm/ps)              100m/s (A/ps)
energy     J (kg-m^2/s^2)  kJ/mol                    kcal/mol 
                             (Da-nm^2/ps^2)            (418.4 Da-A^2/ps^2)
force      N (kg-m/s^2)    kJ/(mol-nm) = TN/mol      kcal/(mol-A)
                             (Da-nm/ps^2) (T=10^12)    (418.4 Da-A/ps^2)

We always keep angles in radians internally, which are unitless. However, most humans prefer degrees where 1 degree = Pi/180 radians so we provide convenient conversions.