From owner-chemistry@ccl.net Sat Aug 22 19:12:00 2015 From: "conor d parks coparks2012-$-gmail.com" To: CCL Subject: CCL: NPT vs. NVT, considerations for crystal simulations and nucleation Message-Id: <-51610-150821171144-27096-7rtQ2Gr4JTnCcVrw8RVEAg(~)server.ccl.net> X-Original-From: "conor d parks" Date: Fri, 21 Aug 2015 17:11:43 -0400 Sent to CCL by: "conor d parks" [coparks2012{}gmail.com] Hello everyone, I was wondering if someone could potentially help me work through a very conceptual problem. I recently was trying to run nucleation simulations of water at 1.00 g/cm^3 217 K in the NVT ensemble. However, I wasn't seeing the types of cluster size fluctuations (I.E. any) I was expecting, and started to wonder if it was due to my use of the NVT ensemble. Now firstly, this isn't due to finite size effects I believe, because the simulation was ran with 2000 water molecules. My issue with water above made me start to think about how I wanted to handle my glycine simulations as well. I am interested in running glycine in water nucleation simulations. For instance, I know glycine's solubility is .338 g/ml at 324 K. So I was going to make my initial box size based off the 0.338 g/ml, equilibrate a fluid at 354 K, and then crash to 270 K. Now I can't seem to figure out if I should run this simulation in NPT or NVT. The solubility data is gathered experimentally at 1 atm, obviously. So if I run NVT, my pressure average will almost assuredly not be 1 atm. However, if I run NPT and fix the pressure at 1 atm, I don't see why I should be guaranteed to fluctuate around the correct solubility. If I think to my undergrad thermo, in the NPT ensemble with a constant number of glycine and water molecules (hence mol fraction is determined), there should be only one equilibrium concentration. I don't see why this equilibrium concentration hopefully generated from an NPT equilibration would correspond to my solubility point. Sorry if that didn't make sense. Conor