return (mass-weighted) metric tensor for given coordinates

Returns :metric_tensor : None or 2d array


The metric tensor is needed for normal mode analysis. In the simplest case it is just the identity. For atomic systems (cartesian coordinates) with masses different to 1.0, the metric tensor is a diagonal matrix with 1/m_i on the diagonal. For curvilinear coordinates like angle axis coordinates it is more complicated. Returning None implies that the metric tensor is the identity.

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