[CP2K-user] Help about AIMD equilibration time for liquid water and water interface simulation

[Shuai Jiang]

Hi, all,

 

Recently I am trying to utilize AIMD in CP2K to equilibrate liquid water 
and water clusters for further analysis. According to the literature, after 
classical MD equilibration, 2 ps in AIMD would be long enough to 
equilibrate the system under the NVT condition with Nose-Hoover thermostat 
(see Fig.1. Input). But under my test, for liquid water (32 water molecules 
composing cluster with PBC, TIP5P equilibrated), even after 5ps, the 
temperature still changes significantly (Fig.1.). So I change the input 
parameters (including decreasing time steps from 1fs to 0.5 fs, decreasing 
the inner scf max cycles while increasing the outer scf cycles, changing 
DFT from BLYP to PBE, increasing EPS_SCF from 1E-5 to 1E-6, changing 
thermostat from Nose to GLE) with the same input configuration, it turns 
out that the most influencing parameter for temperature equilibration is 
thermostat (see Fig.2. for GLE results). It seems like that GLE could 
equilibrate the liquid water faster. But why isn’t Nose-Hoover thermostat 
working in my test? A lot of literature use Nose-Hoover thermostat for 
equilibration and NVE for production.

 

For simulating SO2 interacting with small water cluster surface to test, 
the equilibration time is much longer as the temperature oscillates 
significantly (Fig.3.) even though it is after almost 18 ps equilibration! 
The small water cluster is composed of 24 water molecules and equilibrated 
under TIP3P for 1ns, then the SO2 molecule is put around it, then do the 
geometry optimization with BLYP-D3 till convergence as the initial 
configuration for AIMD. So I am curious about the same question as that in 
simulating the liquid water: why Nose-Hoover thermostat cannot equilibrate 
my system in several ps as indicated in the literature? The faster 
equilibration under GLE thermostat rules out the issue of initial 
configuration, so are there some parameters wrongly setting? Thanks in 
advance.
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