Dear Axel and Stefan,
Many thanks for the answers.
more details, in my problem, a single polymer is confined eg. inside a
sphere. I let the polymer to fluctuate in the sphere for a long time. A
small volume is assumed inside this confining sphere. I need the
simulation time, when one end of the polymer enters this volume. The simulation continues until the end monomers enters the specified volume, 100 times.
I read Sec. 9.1 of the userguide. I do not know which of the opinions below is better.
1. Write the positions of the two end monomers in an output file, then analyze this file with a separate C program. I see in the userguide that observable "particle_positions" is defined and specification of the monomers is possible. However, my simulations are commonly very long, and the output file will be very huge.
2. Write the required simulation times (that end monomers enter the specified volume) in an output file. I do not know if it is possible with the observable command.
Also, I would be grateful if I can have an example on using the command "observable".