The harmonic oscillator approximation for the vibrational energy modes was used: (n+)hv A more realistic model is to include the first term in an expansion for anharmonicity: Evib,n n = 0,1,2, ... ....... fan ) - (n+)v-x(n+ Evib,n hv n = 0,1,2, .... where x is a small constant. What is the vibrational partition function for this case?