[gmx-users] confusion about implicint solvent

[Francesco]

Good afternoon everybody,
I'm a bit confuse about gromacs performances with implicit solvent.

I'm simulating a 1000 residues protein with explicit solvent, using both
a cpu and a gpu cluster.
With a gpu node (12 cores and 3 M2090 gpu ) I reach 10 ns/day, while
with no gpu and 144 cores I got 34 ns/day.

Because I have several mutants (more than 50) I have to reduce the
average simulation time and I was considering different option such as
the use of implicit solvent.
I tried with both the clusters and using gromacs 4.6 and 4.5 but the
performances are terrible (1 day for 100ps) comparing to the explicit
solvent.

I read all the other messages on the mailing-list and the documentation,
but the mix of old and new "features"/posts really confuses me a lot.

Here
(http://www.gromacs.org/Documentation/Acceleration_and_parallelization)
it is said that with the gpu 4.5 and implicit solvent I should expect a
"substantial speedup".

Here (
http://www.gromacs.org/Documentation/Installation_Instructions_4.5/GROMACS-OpenMM#Benchmark_results.3a_GROMACS_CPU_vs_GPU
) I found this sentence "It is ultimately up to you as a user to decide
what simulations setups to use, but we would like to emphasize the
simply amazing implicit solvent performance provided by GPUs."

I follow the advise found in the mailing list and read both the
documentation (site and manual), but I can't figured it out what should
I do.
How can you guys have amazing performances?

I also found this answer from a last March post
(http://gromacs.5086.x6.nabble.com/Implicit-solvent-MD-is-not-fast-and-not-accurate-td5006659.html#none)
that confuses me even more.

"Performance issues are known. There are plans to implement the implicit
solvent code for GPU and perhaps allow for better parallelization, but I
don't know what the status of all that is.  As it stands (and as I have
said before on this list and to the developers privately), the implicit
code is largely unproductive because the performance is terrible. "

Should I skip the idea of using implicit solvent and try something else?

these are a set of parameters that I used (also the -pd flag)

; Run parameters
integrator     = sd    
tinit             = 0        
nsteps         = 50000     
dt                = 0.002   

; Output control
                          
nstxout          = 5000     
nstvout           = 5000   
nstlog             = 5000    
nstenergy       = 5000    
nstxtcout        = 5000  
xtc_precision  = 1000 
energygrps     = system  

; Bond parameters
continuation            = no     
constraints              = all-bonds 
constraint_algorithm = lincs    
lincs_iter                 = 1         
lincs_order              = 4        
lincs_warnangle       = 30      

; Neighborsearching
ns_type                  = simple     
nstlist                     = 0        
rlist                        = 0      
rcoulomb                = 0      
rvdw                      = 0   

; Electrostatics
coulombtype           = cut-off     
pbc                        = no
comm_mode            = Angular

implicit_solvent         = GBSA
gb_algorithm             = OBC
nstgbradii                 = 1.0
rgbradii                      = 0
gb_epsilon_solvent    = 80
gb_dielectric_offset    = 0.009
sa_algorithm             = Ace-approximation
sa_surface_tension    = 0.0054

; Temperature coupling
tcoupl        = v-rescale 
tc_grps      = System
tau_t          = 0.1          
ref_t          = 310        

; Velocity generation
gen_vel        = yes       
ld_seed        = -1    



thank you for the help.

cheers

Francesco


-- 
Francesco Carbone
PhD student
Institute of Structural and Molecular Biology
UCL, London
fra.carbone.12 at ucl.ac.uk