# using a quantity in different steps

Dear all
I want to use and save a quantity (for example “potential energy”) in different time steps in order to do an operation:
for example I want to subtract the value of potential energy when timestep= 1000 from the value of potential energy when time-step = 5000. (ie P.E.@…3811…=5000 - P.E.@…3811…=1000).

i used if command and variable command and used multiple runs but i couldn’t get any success.for example I used:

run 1000
variable pe1 equal pe
run 5000
variable pe2 equal pe

variable deltaPe equal {pe2}-{pe1}
==========END of Input scrip=======
result was: deltaPe = 0

can any one guid me please?

regards

Dear all
I want to use and save a quantity (for example "potential energy") in
different time steps in order to do an operation:
for example I want to subtract the value of potential energy when timestep=
1000 from the value of potential energy when time-step = 5000. (ie
[email protected]...=5000 - [email protected]...=1000).

i used if command and variable command and used multiple runs but i couldn't
get any success.for example I used:

run 1000
variable pe1 equal pe
run 5000
variable pe2 equal pe

variable deltaPe equal \{pe2\}\-{pe1}
==========END of Input scrip=======
result was: deltaPe = 0

as it should be. both equal style variable
expressions are evaluated only when you
define the deltaPe variable.

you have to do such an immediate expansion
when you want to keep the result. an equal style
variable is more like a function than a variable.

# define equal style variable to get the current potential energy
variable getpe equal pe

run 1000
variable pe1 equal \${getpe}

run 5000
variable pe2 equal \${getpe}

# compute the difference between PE after 1000 and after 5000 steps
variable deltaPeTwoOne equal \{pe2\}\-{pe1}

# compute the difference between PE after 1000 steps and current step
variable deltaPe equal v_getpe-\${pe1}