For quick calculations in Mathematica

Some constants that I use almost every time:

V = Quantity[1, "Volts"];
mm = Quantity[1, "Millimeters"];
e = Quantity[1, "ElementaryCharge"];
amu = Quantity[1, "AtomicMassUnit"];
T = Quantity[1, "Teslas"];
Hz = Quantity[1, "Hertz"];