Topography of molecular scalar fields. I. Algorithm and Poincaré–Hopf relation

A new algorithm for locating the critical points (CP’s) of a three-dimensional molecular scalar field is discussed. The algorithm is based on a ray search from the surface extrema of appropriately defined atom-centered spheres. The algorithm is tested for molecular electrostatic potentials and electron densities of a few test molecules such as tetrahedrane, cubane, anthracene, diborane, etc. Furthermore, the Poincaré–Hopf relationship is examined for the set of CP’s thus obtained. A topological interpretation of the Euler characteristic of a given isosurface is employed for a stronger regional check on the number of CP’s enclosed in the isosurface.