# nve and reaxff

Dear all,

I’m doing a simple test of reax/c with nve. I’ve got couple of questions that I want to varify:

1. Etotal is not ‘true’ total energy for reaxff, because as I understand ‘pe’ by default does not include all the potential energy contributions of reaxff, instead, correct total energy is evdwl+ecoul

2. Based on 1, fix nve, which tries to keep total energy (etotal) constant during the run at constant temperature, in case of reaxff shouldn’t be keeping evdwl+ecoul constant?

Manana

Dear all,

I'm doing a simple test of reax/c with nve. I've got couple of questions
that I want to varify:

1. Etotal is not 'true' total energy for reaxff, because as I understand
'pe' by default does not include all the potential energy contributions of
reaxff, instead, correct total energy is evdwl+ecoul

what makes you think this is the case?

2. Based on 1, fix nve, which tries to keep total energy (etotal) constant
during the run at constant temperature, in case of reaxff shouldn't be
keeping evdwl+ecoul constant?

this is nonsense. as has been repeated here many, many times
fix nve does *not* try to "adjust" the total energy. the point of fix
nve is that it *does not do anything* but integrate the equations
of motion. if your system is in equilibrium, then the total energy
*will* be constant. this is the consequence of not adding or removing
energy in any shape or form from the system.

axel.

Dear all,

I’m doing a simple test of reax/c with nve. I’ve got couple of questions
that I want to varify:

1. Etotal is not ‘true’ total energy for reaxff, because as I understand
‘pe’ by default does not include all the potential energy contributions of
reaxff, instead, correct total energy is evdwl+ecoul

what makes you think this is the case?

Here I missed kinetic energy, but anyway, according to manual The thermo variable evdwl stores the sum of all the ReaxFF potential energy contributions. Shouldn’t it coincide with ‘pe’? I might be messing up some things here.

1. Based on 1, fix nve, which tries to keep total energy (etotal) constant
during the run at constant temperature, in case of reaxff shouldn’t be
keeping evdwl+ecoul constant?

this is nonsense. as has been repeated here many, many times
fix nve does not try to “adjust” the total energy. the point of fix
nve is that it does not do anything but integrate the equations
of motion. if your system is in equilibrium, then the total energy
will be constant. this is the consequence of not adding or removing
energy in any shape or form from the system.

axel.

Yes, that’s true. My point here was that if ‘etotal’ and reaxff total energy are not the same than in NVE, what does E is referring to?

Here I missed kinetic energy, but anyway, according to manual The thermo
variable evdwl stores the sum of all the ReaxFF potential energy
contributions. Shouldn't it coincide with 'pe'? I might be messing up some
things here.

the total potential energy should be the pair energy.

> 2. Based on 1, fix nve, which tries to keep total energy (etotal)
> constant
> during the run at constant temperature, in case of reaxff shouldn't be
> keeping evdwl+ecoul constant?

this is nonsense. as has been repeated here many, many times
fix nve does *not* try to "adjust" the total energy. the point of fix
nve is that it *does not do anything* but integrate the equations
of motion. if your system is in equilibrium, then the total energy
*will* be constant. this is the consequence of not adding or removing
energy in any shape or form from the system.

axel.

Yes, that's true. My point here was that if 'etotal' and reaxff total energy
are not the same than in NVE, what does E is referring to?

what does it matter? fix nve doesn't care, so it cannot get it wrong.

axel.