I make a mistake in describing the problem, the obtained snapshot is identical to the first final snapshot, not the final one.
I just open the dump files, and compare the data for final snapshots with those in dump.sivac.final. it’s easy because only 7 atoms are including. The data are as following,
dump.neb.sivac.1:
ITEM: ATOMS id type x y z
174 1 7.08417 9.17725 9.83667
175 1 9.14958 7.11196 9.83674
301 1 7.08424 7.11192 11.9021
304 1 9.1496 9.17732 11.9021
306 1 10.724 8.1566 13.4765
331 1 8.12878 10.7517 13.4764
337 1 10.7239 10.7518 10.8813
dump.neb.sivac.2:
ITEM: ATOMS id type x y z
174 1 7.03601 9.2672 9.78849
175 1 9.23941 7.06381 9.78858
301 1 7.03609 7.06377 11.9919
304 1 8.97297 9.0008 11.7255
306 1 10.6566 8.19209 13.4092
331 1 8.16421 10.6844 13.4091
337 1 10.6565 10.6845 10.9167
dump.neb.sivac.3:
ITEM: ATOMS id type x y z
174 1 6.99256 9.36571 9.74503
175 1 9.33791 7.02036 9.74514
301 1 6.99265 7.02031 12.0904
304 1 8.79677 8.8246 11.5493
306 1 10.6009 8.28329 13.3535
331 1 8.25542 10.6287 13.3534
337 1 10.6008 10.6288 11.0079
dump.neb.sivac.4:
ITEM: ATOMS id type x y z
174 1 6.86846 9.4915 9.62098
175 1 9.46366 6.89643 9.62108
301 1 6.86855 6.89638 12.2162
304 1 8.44288 8.47084 11.1954
306 1 10.5083 8.47088 13.2609
331 1 8.44288 10.5362 13.2608
337 1 10.5082 10.5362 11.1954
dump.sivac.final:
ITEM: ATOMS id type x y z
174 1 7.08417 9.17725 9.83667
175 1 9.14958 7.11196 9.83674
301 1 7.08424 7.11192 11.9021
304 1 9.1496 9.17732 11.9021
306 1 10.724 8.1566 13.4765
331 1 8.12878 10.7517 13.4764
337 1 10.7239 10.7518 10.8813