[gmx-users] Re: Enthalpy Confusion

[Jeffery Perkins]

>>>> or should i be doing < U+<V>*ref_p > = <H>? 
>> 
>>> More specifically, <U> + <V>*ref_p = H 
>> 
>>> <H> isn't really meaningful thing.  I mean, you can define something 
>>> such that <H*> = H, but that's not really thermodynamics. 
>> 
>> sorry I always have issues deciding how to talk about this stuff, so
>> thanks 
>> for putting up with my terrible notation =) 
>> 
>>>> example system gives <H> = -1168 kJ/mol and i find <H> = -725 kJ/mol 
>>>> either 
>> 
>>> Interesting.  What material at what phase conditions?  For liquids, 
>>> the PV contribution should be very small. 
>> 
>> I hadn't really thought about that... but the system is a monatomic 
>> Lennard-Jones particle (uncharged sigma = 0.35 nm epsilon = 2 kJ/mol mass
>> = 
>> 40 amu) which should be a liquid at the conditions I was looking at,
>> P=1000 
>> bar, T = 300 K using phase diagram in: Equation of state for the 
>> Lennard-Jones fluid, J. J. Nicolas et al., MOLECULAR PHYSICS, 1979, VOL.
>> 37, 
>> No. 5, 1429-1454 
>> 
>> the <U> is -1600 and the p<V> is 880 when manually done, around 400 from 
>> g_energy 
>Here's the code: 
>             /* This is pV (in kJ/mol).  The pressure is the reference
> pressure, 
>                not the instantaneous pressure */ 
>             pv = vol*md->ref_p/PRESFAC; 
>
>             add_ebin(md->ebin, md->ipv, 1, &pv, bSum); 
>             enthalpy = pv + enerd->term[F_ETOT]; 
>
>What is your volume? 
>What is Etot?

a bit messy but this is data from g_energy for the system with increasing
temperature (300 K-1000 K), and constant pressure of 1000 bar (so 100 000
000 Pascals):

Temperature	Volume    Enthalpy	Potential	Total Energy	pV        	Kinetic En.
K                     nm^3       kJ/mol            kJ/mol                
kJ/mol           kJ/mol                 kJ/mol
299.914		14.7226  -1168.83	-2565.44	-1611.63	442.791 	953.817
399.901		17.462   -339.795	-2080.31	-808.501	468.706 	1271.81
499.898		21.0263   421.763	-1666.7	            -76.8794	498.643           
1589.83
599.899		25.1149   1088.79	-1348.14	559.718 	529.07   	1907.86
699.914		29.4219   1668.2	-1115.47	1110.46 	557.733 	2225.94
799.925		33.764    2184.31	-943.613	1600.39 	583.922 	2544
899.923		38.0865  2660.3 	-809.572	2052.45 	607.847 	2862.03
999.931		42.2798  3101.56	-707.908	2472.17 	629.383 	3180.08

all uncorrected for the number of particles in the box (so Etot is in kJ/256
mol of particles)

when i use this data to get H myself i get:

H
kJ/mol
-725.32948
242.7114
1188.90386
2071.63498
2881.65838
3632.9828
4345.2573
5017.41396

again uncorrected for the number of particles in the box

Thoughts?



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