Hello All,

My purpose is not to annoy people. Please ignore my email if you feel

annoyed.

it would be less annoying, if you would

pay better attention to what you got told.

I am working on a problem in which I need to determine exact value of

enthalpy that is of the order of 10- 60 meV aka 0.01 - 0.06 eV with applied

pressure. In case of no external pressure, the energy should be equal to

i already explained, that with *any* numerical method,

exact identity is an impossibility. the *exact* value of

your total energy *will* depend on the order of how it

is being summed up, since addition and and multiplication

in floating point math do *not* commute. so to expect

identity is ignoring fundamental properties of the

method you are using, to say the least.

enthalpy. MD method is giving the desired results and now I am looking for

options within CG / SD if possible.

so if you get what you want with simulated annealing,

why bother with minimization? as i wrote before, you

are trying to converge a property beyond a meaningful

threshold. have you considered trying to understand

why rather than insisting on something that you got

told to be pointless?

I am working on suggestions given by the group and update the group about

output. I understand the limitations of different operating system etc.

no, you don't understand. this has *nothing* to do

with the operating system, but is *general* issue

of the propagation of truncation errors when doing

floating point math.

Also, I am relatively new to atomistic simulations.

so, why ignore good advice?

Seems like the discrepancy of energy and enthalpy comes from residual

forces. Using CG or SD, the energy is minimized even though the residual

forces are not zero locally somewhere in the simulations cell.

I tried the following options with CG and SD

min_style cg

min_modify line quadratic

This gives the same discrepancy : Energy is not equal to enthalpy

but by how much?? identity is impossible.

you have only at the most 17 digits accuracy

in double precision floating point math. since

you are summing up derived properties of

different magnitude, probably significantly less.

min_style cg

min_modify line forcezero

This option gives : "ERROR Illegal min_modify command" Why ?

i *already* told you: because you have an outdated

or miscompiled LAMMPS executable.

min_style sd

min_modify line quadratic

result = Energy is not equal to enthalpy

again, by how much?

min_style sd

min_modify line forcezero

This option gives : "ERROR Illegal min_modify command" . Why ?

I would love to hear about what needs to be modified for using "min_modify

line forcezero". Also, whether CG or SD are capable of solving my problem or

I need to stick to MD method ?.

primarily, you need to rethink what is the meaning of error

and accuracy when doing floating point math. insisting on

getting bitwise identical results with these two scenarios

is ignoring the realities of computer simulations. perhaps

you should port LAMMPS to "bc", which can do arbitrary

precision math (at a cost, of course).

axel.