N,D,0,……} corresponds that every molecule is in the ground state, there is only one way to achieve this configuration; N-2, 2, 0,.o.3 corresponds that two molecule is in the first excited state and the rest in the ground state and can be achieved in N(N-1)/2 ways A configuration (no, nu, n 2,…… can be achieved in w different ways, where w is called the weight of the configuration And w can be evaluated as follows W=M!/(mnln1n2…){N, 0, 0, ......} corresponds that every molecule is in the ground state, there is only one way to achieve this configuration; {N-2, 2, 0, ......} corresponds that two molecule is in the first excited state, and the rest in the ground state, and can be achieved in N(N-1)/2 ways. A configuration { n0 , n1 , n2 , ......} can be achieved in W different ways, where W is called the weight of the configuration. And W can be evaluated as follows, W = N! / (n0 ! n1 ! n2 ! ...)