Molecular normal coordinate
= Molecular normal coordinate
{title2=$Q_i$}
A mass-weighted displacement coordinate along an eigenvector of the molecular harmonic force-constant matrix after rigid translations and rotations are removed. An orthonormal normal-mode basis gives kinetic energy $\frac12\sum_iP_i^2$ and harmonic potential $\frac12\sum_i\omega_i^2Q_i^2$. This use of normal coordinate concerns molecular vibrations, rather than normal coordinates of a spacetime metric.