Simbody
3.6
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Modules | |
Mathematical Constants | |
Physical Constants | |
Unit Conversion Factors | |
These are preprocessor (#define) macros providing constants values at very high precision. See the discussion under the main Predefined Constants module heading.
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.