Entropy of Reissner-Nordstr\"om-de Sitter black hole

Based on the consideration that the black hole horizon and the cosmological horizon of Reissner-Nordstr"om black hole in de Sitter space are not independent each other, we conjecture the total entropy of the system should have an extra term contributed from the entanglement between the two horizons, except for the sum of the two horizon entropies. Making use of the globally effective first law and the effective thermodynamic quantities, we derive the total entropy and find that it will diverge as the two horizons tends to coincide.