12
This conformation of trans-2-butene has C2h symmetry
 1   0.030847952   2.333333770  -2.090768623
 6   0.030847875   2.333333770  -0.994768685
 6   1.184367647   2.333333770  -0.328783491
 6  -1.295036904   2.333333770  -0.229268451
 6   2.510252729   2.333333770  -1.094283201
 1   1.184367108   2.333333770   0.767216492
 1   3.343300183   2.333333770  -0.382069813
 1   2.568310810   3.228213946  -1.724390076
 1   2.568310810   1.438453595  -1.724390076
 1  -2.128084646   2.333333770  -0.941481500
 1  -1.353094737   3.228213946   0.400838416
 1  -1.353094737   1.438453595   0.400838416
