Quantum geometry of connected state manifolds: When diabolic points act as bridges between eigenstate manifolds

Parametric Hamiltonians often exhibit point-like spectral degeneracies (diabolic points, or conical intersections), which can lead to singularities in the Provost-Vallee metric of eigenstate manifolds. We regularise the metric by a coordinate transformation and develop a formalism in which diabolic points act as bridges between adjacent eigenstate manifolds, glueing them into a single connected state manifold. We characterise the topology of this structure and refine the rules for nodal lines governing the Berry phase. The connected state manifold restores the numerical stability near diabolic points, enlarges the class of geodesics allowing for new geodesic shortcuts, and provides a new mechanism for Berry phase computation, even along paths traversing diabolic points.