  We present a straightforward method for obtaining exact classical and quantum
molecular Hamiltonians in terms of arbitrary coordinates. As compared to other
approaches the resulting expression are rather compact, the physical meaning of
each quantity is quite transparent, and in some cases the calculation effort
will be greatly reduced. We also investigate systems with constrains to find
the suggested method to be applicable in contrast to most conventional
approaches to kinetic energy operators which cannot directly be applied to
constrained systems. Two examples are discussed in detail, the monohydrated
hydroxide anion and the protonated ammonia dimer.
