Because there can be several different potentials in the same program, there can be more than one boundary condition type. Trying to solve this problem, a node to the list, called BCNode, has been created, containing the basic data to the bound'ary condition definition. The characteristics are inherited from this class to form specific boundary conditions, that is, classes with proper characteristics (e.g., vector potential) are descendants of BCNode, with the add of their particular data.