在Gaussian的高级输入文件中，经常可以看到如下格式的IOP指令： Iop（n1/n2=n3) 说到底，Iop就是Gaussian程序内部的一些高级指令，从更加底层设置程序运行的各种参数。gaussian是个傻瓜型软件，所以很多比较深入的细节都要通过IOP来设置。上述指令中，n1表示overlay，n2表示指令序号，n3表示n1/n2指令的值。具体的IOP的overlay和指令号及所对应值的含义可以在Gaussian网上的IOP大全上找到。具体网址为：
http://www.gaussian.com/iops.htm 比如有IOP(2/15=3)的指令， 则我们找到Overlay2的第15个指令序号，点击进入后可以看到如下的信息 IOp(2/15) SYMMETRY CONTROL. -1 Turns on symmetry; same as 0 for molecules but turns on assignment of space group ops. for PBC. 0 Leave symmetry in whatever state it is presently in. 1 Unconditionally turn symmetry off. Note that symm is still called, and will determine the framework group. However, the molecule is not oriented. 2 Bring the molecule to a symmetry orientation, but then disable further use of symmetry. 3 Don't even call symm. 4 Call Symm once with loose cutoffs, symmetrize the resulting coordinates, then confirm symmetry with tight cutoffs. 5 Recover the previous symmetry operations from the rwf, and confirm that the new structure has the same symmetry. 6 Same as 5, but get symmetry info from the chk. 00 Default (10) 10 Do re-orientation for PBC. 20 Suppress re-orientation for PBC. 100 Turn on symmetry operations for PBC. 由于我们将其值设为3，则表明计算中不要调用对称性判定的程序。
另外常用的IOP的设置有 Iop(5/33=3,3/33=1) 用来输出Fock矩阵和overlap矩阵。
