def is_valid(self, tol: float = DISTANCE_TOLERANCE) -> bool:
        """
        True if SiteCollection does not contain atoms that are too close
        together. Note that the distance definition is based on type of
        SiteCollection. Cartesian distances are used for non-periodic
        Molecules, while PBC is taken into account for periodic structures.

        Args:
            tol (float): Distance tolerance. Default is 0.5A.

        Returns:
            (bool) True if SiteCollection does not contain atoms that are too
            close together.
        """
        if len(self.sites) == 1:
            return True
        all_dists = self.distance_matrix[np.triu_indices(len(self), 1)]
        return bool(np.min(all_dists) > tol)