Hi everyone,

I have a question about ab initio molecular dynamics. What is the difference between AIMD_INIT_VELOC=ZPE and AIMD_INIT_VELOC=QUASICLASSICAL?

I am planning to perform a quasiclassical direct dynamics trajectory simulation on a transition structure. However, when I use AIMD_INIT_VELOC=QUASICLASSICAL on qchem 6.2, I get an error message stating “Quasiclassical MD not yet supported for transition states.” Conversely, the simulation runs successfully with AIMD_INIT_VELOC=ZPE.

input:

```
$molecule
0 2
C 3.3135997804 -0.0078810290 -0.0093651548
H 3.6066735648 0.7959221128 -0.6856021114
H 3.6363870152 0.1567299549 1.0199545771
H 3.5760919013 -0.9973732332 -0.3883608468
C 0.6323632532 0.0166113714 0.0131679721
C -0.0441388971 1.2281272323 0.0146353683
C -1.4435978962 1.2107274345 0.0042023458
C -2.1267743693 -0.0075961871 -0.0077880088
C -1.4224439670 -1.2139181757 -0.0095801812
C -0.0230120434 -1.2062566659 0.0006442183
H 2.0065471467 0.0174336366 0.0153133373
H 0.4981169822 2.1708343243 0.0233488537
H -1.9991640785 2.1452594740 0.0050702665
H -3.2145949435 -0.0169757986 -0.0163280596
H -1.9618313722 -2.1578670589 -0.0193435608
H 0.5368126337 -2.1388548629 -0.0015363369
$end
$rem
JOBTYPE freq
METHOD m06
BASIS 6-31g
scf_convergence 12
$end
@@@
$molecule
read
$end
$rem
JOBTYPE aimd
METHOD m06
SCF_CONVERGENCE 6
TIME_STEP 20 ! (in atomic units)
AIMD_STEPS 1000 ! 600 fs total simulation time
AIMD_TEMP 12
AIMD_PRINT 2
FOCK_EXTRAP_ORDER 6 ! Use a 6th-order extrapolation
FOCK_EXTRAP_POINTS 12 ! of the previous 12 Fock matrices
AIMD_MOMENTS 1
AIMD_NUCL_SAMPLE_RATE 5
AIMD_NUCL_VACF_POINTS 1000
AIMD_INIT_VELOC QUASICLASSICAL
AIMD_QCT_WHICH_TRAJECTORY 1 ! Loop over several values to get
BASIS 6-31g
MAX_SCF_CYCLES 1000
SCF_ALGORITHM DIIS_GDM
$end
```