[gmx-users] the principle of pulling

[戏梦]

Dear,  i am doing some simulation about pulling alkane into my micelle in umbrella sampling. i want to ask somethin about the output result in the pullf.xvg file. In the file, there are two lines data, one is time and the other is force.  I want to know how the force come? or is there any formula about calculating the force? And, I do not understant the principle about the pulling in GMX after I learing the manual. Is there any literature or book about the pulling. Thanks in advance!