COMPARISONS between the results of accurate quantum-mechanical calculations on atoms and molecules have been made unnecessarily complicated and sometimes even impossible by the lack of an agreed and unambiguous system of units. A calculated binding energy, for example, may well be reported in kcal./mol. or eV. on one hand, or in Rydbergs or Hartree atomic units (double Rydbergs) on the other. The latter pair have the advantage of being the primary result of the calculation but the disadvantage of being frequently denoted by the same cumbersome symbol a.u. Furthermore, the same symbol is also used for a unit of length (the first Bohr radius) for a unit of electric moment and, in quantum field theory, for a quite different energy. To compound the confusion, some authors have even used double Hartree a.u. (quadruple Rydbergs). Then, in addition, atomic units usually leave unspecified whether or not a reduced mass is given unit value instead of the electron mass, or indeed whether other modifications have been made (see below). Even when the modification of one unit is specified, its effect on the other units may still be quite uncertain until it is agreed which units are basic and which are derived from them.