public class Units {
  // Time (base units are seconds)
  static final double femtoseconds = 1.0e-15;
  static final double picoseconds = 1.0e-12;
  static final double nanoseconds = 1.0e-9;
  static final double microseconds = 1.0e-6;
  static final double milliseconds = 1.0e-3;
  static final double seconds = 1.0;

  // Distance (base units are meters)
  static final double angstroms = 1.0e-10;
  static final double nanometers = 1.0e-9;
  static final double meters = 1.0;

  // Energy (base units are Joules).
  static final double joules = 1.0;
  static final double kilocalories_per_mole = 4184.0 / Constants.NA;

  // Temperature (base units are Kelvin).
  static final double kelvin = 1.0;

  // Mass (base units are kg).
  static final double kilograms = 1.0;
  static final double grams = 1.0e-3;
  static final double amu = grams / Constants.NA;
}