angstrom = 1.00001501 * 10^-10 meter;
aulength = 0.5291772083*10^-10 meter;
auenergy = 4.35974381 * 10^-18 joule;
invmeter = 1.98644544 * 10^-25 joule;
invmeter1 = 4.556335252750 * 10^-8 hartree;
aulengthinangstrom = aulength/angstrom;
auenergyininvcm = auenergy/invmeter/100;
(* unified atomic mass unit, or atomic mass constant *)
u = 1.66053873 * 10^-27 kg;
elmass = 9.10938188 * 10^-31 kg;
uinelmass = u/elmass;

(* see also http://www.gaussian.com/00000486.htm *)