Problem with fluid/fluid interaction parameter with velocity condition in channel flow

Dear All,

I am trying to simulate flow through 3 D channel using shan-chen model. I am giving Zou-He boundary condition at inlet and outlet. whenever i am giving the value of interaction parameter g_12=g_21=0, the code is working fine but for any other value of g_12, g_21, the code is giving NAN. Can anyone help me out in this regard?