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(May 16, 2021)

The potential energy landscape of atomic and molecular clusters can be very
complex with many minima which can have similar energies, and this complexity
increases rapidly as the size of the clusters increases. Determining the global
minimum of these clusters is challenging since it requires extensive searching
over the potential energy surface. One approach to finding the low energy
structures of these clusters is to perform many geometry optimisations starting
at different initial coordinates. Q-Chem is able to perform such random
searches for molecular clusters containing up to two different molecule types.
In these searches the molecules are subjected to translations and rotations of
their structure to generate a new starting structure. These searches are
initiated by the JOBTYPE = RAND and it is necessary to
specify the number of molecules of the different types and the number of atoms
in the different types of molecules. For the optimisation of atomic clusters,
SEARCH_ATOMIC = TRUE and the number of atom swops performed
in the structure generation (N_SWOP) can be specified. Some care has
to be taken with the specification of the input structure in the
*$molecule* section. All the atoms of the molecules of molecule type 1
must come before those of molecule type 2. Furthermore, the atoms of the same
molecule should be together. For examples of these studies see
273, 274, 275, 675.

$molecule 1 1 N 0.5682008336 0.1585044954 -0.9009280260 O -0.3450383302 -0.5598328271 -0.4634299478 O 1.7303273568 0.3403569345 0.4364171165 H 2.5236300547 -0.2494576134 0.1485689942 H 2.1020812302 1.2823911654 0.2570156558 $end $rem JOBTYPE RAND METHOD B3LYP BASIS STO-3G SCF_CONVERGENCE 6 MAX_SCF_CYCLES 100 NSEARCH 10 N_MOL_TYPE 2 NMOL1 1 N_ATOM_TYPE_1 2 NMOL2 1 N_ATOM_TYPE_2 3 N_MOVES 20 MAXBOX 10000 MIN_SEPARATION 25 MAX_DISPLACE 25 SCF_NOCRASH TRUE TIGHTEN_CONVERG TRUE GEOM_OPT_MAX_CYCLES 200 GEOM_OPT_COORDS 0 GEOM_OPT_TOL_DISPLACEMENT 1000 GEOM_OPT_TOL_GRADIENT 300 GEOM_OPT_TOL_ENERGY 100 $end

$molecule 0 1 He -1.3590894 3.0177788 -0.1662522 He -2.9853158 1.1444488 0.1036005 He 0.5068109 1.3795209 -0.2168151 Ne -1.1002149 -0.5693061 0.0381894 Ne 0.5981676 1.8697812 1.4685618 Ne -1.2376457 1.2597811 -0.0756066 $end $rem JOBTYPE RAND METHOD B3LYP DFT_D EMPIRICAL_GRIMME BASIS STO-3G SCF_CONVERGENCE 6 MAX_SCF_CYCLES 100 NSEARCH 10 SEARCH_ATOMIC TRUE N_SWOP 4 N_MOL_TYPE 2 NMOL1 3 N_ATOM_TYPE_1 1 NMOL2 3 N_ATOM_TYPE_2 1 N_MOVES 20 MAXBOX 10000 MIN_SEPARATION 25 MAX_DISPLACE 25 SCF_NOCRASH TRUE TIGHTEN_CONVERG TRUE USE_INITIAL TRUE GEOM_OPT_MAX_CYCLES 200 GEOM_OPT_COORDS 0 GEOM_OPT_TOL_DISPLACEMENT 1000 GEOM_OPT_TOL_GRADIENT 3000 GEOM_OPT_TOL_ENERGY 1000 $end

Basin hopping (BH)
^{
1144
}
J. Phys. Chem. A

(1997),
5111,
pp. 101.
Link
is a more advanced technique for locating
the global minimum on complex potential energy surfaces. The BH algorithm is
essentially a combination of the Metropolis sampling technique and a
gradient-based local search method. This has the effect of sampling the energy
basins, where an energy basin is a certain part of the configuration space
around a minimum on the PES that contains all the configurations that will
relax into this minimum using downhill relaxations, instead of sampling the
configuration space. To enhance the efficiency of the method, BH with
occasional jumping
^{
489
}
Chem. Phys. Lett.

(2004),
399,
pp. 396.
Link
is used, which incorporates a jumping
move in addition to the standard Monte Carlo (MC) moves. Jumping is a MC move
without local minimization at infinite temperature and, consequently, is always
accepted. When the usual MC moves are rejected a number of times, the system
is judged to be trapped at the local minimum. The temperature is raised to T =
$\mathrm{\infty}$, and the MC jumping moves are executed several times to allow the
system to escape from the local minimum. This provides an efficient way to
escape from a local minimum and to explore the next basin of the valley when it
is separated by high barriers. Depending on the size and complexity of the
system being studied, a large number of MC_STEPS and/or
MC_CYCLES to ensure the global minimum is found.

$molecule 1 1 N 0.5682008336 0.1585044954 -0.9009280260 O -0.3450383302 -0.5598328271 -0.4634299478 O 1.7303273568 0.3403569345 0.4364171165 H 2.5236300547 -0.2494576134 0.1485689942 H 2.1020812302 1.2823911654 0.2570156558 $end $rem JOBTYPE BH METHOD B3LYP BASIS STO-3G SCF_CONVERGENCE 6 MAX_SCF_CYCLES 100 MC_CYCLES 4 MC_STEPS 5 MC_TEMP 300 MAX_DISPLACE 25 MIN_SEPARATION 25 MAXBOX 5000 N_MOVES 20 N_MOL_TYPE 2 NMOL1 1 N_ATOM_TYPE_1 2 NMOL2 1 N_ATOM_TYPE_2 3 N_MOVES 20 MAXBOX 10000 MIN_SEPARATION 25 MAX_DISPLACE 25 SCF_NOCRASH TRUE GEOM_OPT_MAX_CYCLES 200 GEOM_OPT_COORDS 0 GEOM_OPT_TOL_DISPLACEMENT 2000 GEOM_OPT_TOL_GRADIENT 4000 GEOM_OPT_TOL_ENERGY 400 $end

NSEARCH

INTEGER

TYPE:

Sets the number of structures that are generated and optimised.

DEFAULT:

No default.

OPTIONS:

User defined.

RECOMMENDATION:

None

SEARCH_ATOMIC

Perform an optimisation for atomic cluster.

TYPE:

BOOLEAN

DEFAULT:

False

OPTIONS:

True
Atomic cluster search will be performed.
False
Molecular clusters search will be performed.

RECOMMENDATION:

Use N_SWOP to specify atomic number of atom swops in structure generation.

N_SWOP

INTEGER

TYPE:

Sets the number atom coordinate swops for atomic cluster search.

DEFAULT:

No default.

OPTIONS:

User defined

RECOMMENDATION:

None

N_MOL_TYPE

INTEGER

TYPE:

Sets the number of different atom/molecule types.

DEFAULT:

No default.

OPTIONS:

User defined : can be 1 or 2.

RECOMMENDATION:

None

NMOL1

INTEGER

TYPE:

Sets the number of molecules of type 1.

DEFAULT:

No default.

OPTIONS:

User defined.

RECOMMENDATION:

None

N_ATOM_TYPE_1

INTEGER

TYPE:

Sets the number atoms in molecule type 1.

DEFAULT:

No default.

OPTIONS:

User defined.

RECOMMENDATION:

None

NMOL2

INTEGER

TYPE:

Sets the number of molecules of type 2.

DEFAULT:

No default.

OPTIONS:

User defined.

RECOMMENDATION:

None

N_ATOM_TYPE_2

INTEGER

TYPE:

Sets the number atoms in molecule type 2.

DEFAULT:

No default.

OPTIONS:

User defined.

RECOMMENDATION:

None

MAXBOX

Sets the size of the box which the molecules are kept within.

TYPE:

INTEGER

DEFAULT:

20000

OPTIONS:

$n$
Corresponding to MAXBOX = $n/1000$ bohr.

RECOMMENDATION:

Need to ensure that the cluster can fit within this box.

MIN_SEPARATION

Reject initial structures where the closest approach of molecules is less than this value.

TYPE:

INTEGER

DEFAULT:

300

OPTIONS:

$n$
Corresponding to MIN_SEPARATION = $n/100$ bohr.

RECOMMENDATION:

MIN_SEPARATION of approximately 2.5 bohr.

MAX_DISPLACE

Sets the maximum distance a molecule will be moved during a translation.

TYPE:

INTEGER

DEFAULT:

500

OPTIONS:

$n$
Corresponding to MAX_DISPLACE = $n/100$ bohr.

RECOMMENDATION:

None.

SCF_NOCRASH

Ensure the calculations continues if the SCF fails to converge for a given structure.

TYPE:

BOOLEAN

DEFAULT:

False

OPTIONS:

True
Ensure calculation will continue with next structure.
False
Calculation will stop.

RECOMMENDATION:

Use SCF_NOCRASH = TRUE

TIGHTEN_CONVERG

At the end of the search re-calculate the energies of the optimised structures with
tighter SCF convergence criteria.

TYPE:

BOOLEAN

DEFAULT:

False

OPTIONS:

True
Additional calculations with tighter SCF convergence performed.
False
No additional calculations performed.

RECOMMENDATION:

None.

USE_INITIAL

Include input structure as part of the search.

TYPE:

BOOLEAN

DEFAULT:

False

OPTIONS:

True
Input structure is included in the search.
False
Input structure is not included in the search.

RECOMMENDATION:

None.

SEARCH_MOM

Allows the search to be performed in conjunction with MOM to explore excited states.

TYPE:

BOOLEAN

DEFAULT:

False

OPTIONS:

True
A search with MOM is performed.
False
Normal calculation without MOM.

RECOMMENDATION:

None.

MC_CYCLES

INTEGER

TYPE:

Sets the number of cycles in a basin hopping search.

DEFAULT:

No default.

OPTIONS:

User defined.

RECOMMENDATION:

None

MC_STEPS

INTEGER

TYPE:

Sets the number of Monte Carlo steps in each MC_CYCLES. After
MC_STEPS jumping is initiated.

DEFAULT:

No default.

OPTIONS:

User defined.

RECOMMENDATION:

None

MC_TEMP

INTEGER

TYPE:

Sets the temperature (in Kelvin).

DEFAULT:

300

OPTIONS:

User defined.

RECOMMENDATION:

None

N_MOVES

INTEGER

TYPE:

Sets the number of structural changes/moves on each step.

DEFAULT:

2

OPTIONS:

User defined.

RECOMMENDATION:

None

MAX_JUMP

INTEGER

TYPE:

Sets the number of moves accepted on jumping.

DEFAULT:

10

OPTIONS:

User defined.

RECOMMENDATION:

None