Hello Riping. This error is due to the Gibbs phase rule, which says F=C-P+2, where F is the total number of degrees of freedom in the system, C is the number of components, and P is the number of phases. In a system of pure water with a liquid and vapor phase, C=1 and P=2, so F=1. That is, you can only have a single degree of freedom where both phases coexist. This is generally the temperature, so for a single component system with two phases, you have to use the NVT Gibbs ensemble. If you want to use NpT (which has two degrees of freedom, the temperature and the pressure), and you have two phases (two simulation boxes), you have to have at least two components in your system. <div> </div><div> Cheers, Matt</div><div> On Tuesday, September 18, 2012 8:29:47 PM UTC-5, Northstar wrote:<blockquote class="gmail_quote" style="margin: 0;margin-left: 0.8ex;border-left: 1px #ccc solid;padding-left: 1ex;">Dear matt, I concern on change A defined above. I follow your instruction to make the second box. box1 and box2 have BOX2_FILE_NAME linked to each other. It comes the following error. ''' ************************************************************ *** ERROR in mc_input_parameters_check (MODULE mc_types) *** ************************************************************ *** Cannot have GEMC-NPT simulation with only one molecule type *** *** Program stopped at line number 2068 of MODULE mc_types *** ===== Routine Calling Stack ===== 1 CP2K CP2K| Abnormal program termination, stopped by process number 0 ''' And then I adjust AVBMC_ATOM 1 to be AVBMC_ATOM 3 in both input files. The following error comes. ''' ************************************************************ *** ERROR in mc_input_parameters_check (MODULE mc_types) *** ************************************************************ *** Cannot have GEMC-NPT simulation with only one molecule type *** *** Program stopped at line number 2068 of MODULE mc_types *** ===== Routine Calling Stack ===== 1 CP2K CP2K| Abnormal program termination, stopped by process number 0 ''' Is it any help for me? Thank you very much. WANG Riping 2012.9.19 </blockquote></div>