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$LIBE Roundoff problems

Alexander V. Malutin
engager6@mail.ru

Hi. I'm constantly getting bored by strange problems with $LIBE group. Every time i try to optimize molecule with linear bended region in internal coordinates, geometry search is almost constantly failing to locate stationary point then i'm using OPTTOL=1d-5 or below. Visualization of search steps shows oscillatory behavior near linear bend geometry, so i'm pretty much convinced that using linear bend $LIBE group is essential. However then i do use it, it results in error on the first step:

         TRANSFORMING DISPLACEMENT FROM INTERNALS TO CARTESIANS
         THE ROOT MEAN SQUARE ERROR IN ITERATION   1 IS   0.00004979
         THE ROOT MEAN SQUARE ERROR IN ITERATION   2 IS   0.00006993
         THE ROOT MEAN SQUARE ERROR IN ITERATION   3 IS   0.00013865
*** ROUNDOFF ERROR IN LIBE - TEST,DOTP=   -0.2563072398E-06   -0.7003238419E-03

How can i solve this problem? May be i am missing something and should supply input files with some special keys to avoid such errors? As far as i can see such errors have unpredictable character, sometimes $LIBE supported input works just fine, but very often run fails due this nasty roundoff problem, witch is extremely annoying. Will grateful for any help regarding this matter.

Best regards
Alexander

Input file:

$SYSTEM MEMORY=128000000 $END
$CONTRL SCFTYP=MCSCF RUNTYP=OPTIMIZE $END
$CONTRL UNITS=ANGS MULT=1 NZVAR=8 $END
$BASIS GBASIS=N31 NGAUSS=6 NPFUNC=1 NFFUNC=1 $END
$MCSCF CISTEP=GUGA FULLNR=.TRUE. FORS=.FALSE. $END
$DRT GROUP=C1 SOCI=.TRUE. NMCC=5 NDOC=6 NVAL=3 NEXT=2 $END
$STATPT NSTEP=100 OPTTOL=1d-6 HSSEND=.TRUE. $END
$FORCE NVIB=2 VIBANL=.TRUE. PURIFY=.TRUE. TEMP(1)=0.001, 298.15, 398.15 $END
$GUESS GUESS=MOREAD NORB=67 $END
$DATA
Cyanamide NH2-CN MCSCF.
C1
N           7.0   0.0000000000   0.0000000001   1.2870668159
C           6.0   0.0000000001   0.0000000033   0.1424955716
N           7.0   0.0000000000  -0.0000000042  -1.1681804833
H           1.0  -0.0000000001   0.8498848363  -1.6742591359
H           1.0  -0.0000000001  -0.8498848188  -1.6742591918
$END
$ZMAT IZMAT(1)=1,1,2, 1,2,3, 1,3,5, 1,3,4,
2,2,3,5, 2,5,3,4, 3,2,3,4,5, 5,1,2,3 $END
$LIBE APTS(1)=1.0,0.0,0.0 $END
$VEC
....
....
....
$END  

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