***, Allene geometry optimization using natural internal coordinates
 basis=sto-3g

 rcc=1.32 ang
 rch=1.08 ang
 acc=120  degree
 symmetry,nosym
 Geometry={C1;                       !Z-matrix input
           C2,c1,rcc
           Q1,c1,rcc,c2,45
           C3,c2,rcc,c1,180,q1,0
           h1,c1,rch,c2,acc,q1,0
           h2,c1,rch,c2,acc,h1,180
           h3,c3,rch,c2,acc,h1,90
           h4,c3,rch,c2,acc,h2,90}
 hf;
 optg                !default optimization using model hessian
 coord,bmat          !use natural internal coordinates

 optg,coord=bmat     !same as above