General section

In this section the electronic structure details are provided. The following is an example of the keywords employed in this section for the FA molecule.

--General--  
molecule       FA  
LowLevel       mopac pm7 t=3m  
HighLevel      g16 b3lyp/6-31G(d,p)  
HL_rxn_network complete  
IRCpoints      30  
charge         0  
mult           1
timeout        600

List of Keyword value(s) for this section:

molecule value
[value is one string with no blank spaces; mandatory keyword]

value is the name of the system and it must match the name of the XYZ file without the extension, FA in our example. For association and vdW sampling there is no XYZ file at the beginning and value is just the name of the system.

LowLevel values
[two values: two strings; the second string accepts blank spaces; default: mopac pm7]

The first value is the program and the second the semiempirical method. So far, qcore and mopac are valid programs. For qcore only xtb method is implemented, and for mopac, any of the semiempirical methods of MOPAC2016 can be employed to run the MD simulations. You can use a combination of MOPAC keywords. In the example above, for instance, the pm7 semiempirical level together with a maximum CPU time, for any type of mopac calculation, of 3 minutes is requested. The use of the MOPAC keyword t=, followed by an amount of time, is highly recommended to enhance the efficiency of the calculations.

If you do not employ the keyword LowLevel_TSopt, explained below in advanced options, both the low-level TS optimizations and MD simulations are carried out using the semiempirical method specified by the second value. This is in general a good choice both in terms of efficacy and efficiency, and also because all structures will be re-optimized later using ab initio/DFT methods as specified with the keyword HighLevel.

However, if you know that semiempirical methods do not work well for your system, and although they are going to be employed for the MD sampling, you can still pick one of the ab initio/DFT methods already at this stage for the TS optimizations using the keyword LowLevel_TSopt explained below in advanced options. However, note that this will be much more demanding.

HighLevel values
[two values: two strings; no blank spaces in each string; mandatory keyword except for association]

The first value is the program (g09, g16 or qcore are the possible choices) and the second indicates the level of theory employed in the high-level calculations. For gaussian, you can employ a dual-level approach, which includes a higher level to refine the energy, as shown in the following example:

HighLevel g16 ccsd(t)/6-311+G(2d,2p)//b3lyp/6-31G(d,p)

For g09/g16, supported methods are HF, MP2 and DFT for geometry optimizations and HF, MP2, DFT and CCSD$($T$)$ for single point energy calculations.

For qcore, the method is specified in an additional file named qcore_template. An example of such file is given in the FA_qcore example. This option also allows an extra keyword: hessianmethod, which could accept the values analytic or semianalytic.

HL_rxn_network value(s)
[one or two values: first is a string, and second, if present, is an integer; default value: reduced]

The first value can be complete or reduced. complete indicates that all the TSs will be reoptimized and in this case no second value is needed.

Alternatively, you may use reduced as the first value, the default, followed by a second value, an integer, which indicates the maximum energy, in kcal/mol and relative to the reference starting structure, of a transition state to be calculated at the high level.

IRCpoints value
[value is an integer; default value: 100 ]

value is the maximum number of IRC points in each direction computed at the high-level. Note that g09/g16 calculations need much fewer points than Entos Qcore.

charge value
[value is an integer; default value: 0 ]
value is the charge of the system.

Memory value
[value is an integer; default value: 1 ]
value is the number of GB of memory employed in the gaussian high-level calculations.

mult value
[value is an integer; default value: 1 ]
value is the multiplicity of the system. Note that this keyword is only employed in the HL calculations. If you want to run the LL calculations with a specific multiplicity, this should be specified in the LowLevel keyword using any of the possibilities that MOPAC offers.

timeout value
[value is an integer; default value: 1000000 ]
value is the maximum time, in seconds, allowed for a parallel job to complete. This helps terminate lengthy jobs that are likely to fail and act as bottlenecks in parallel compuations. This keyword is similar to MOPAC’s t= keyword, but it is more general and can be applied to any job.

The user is advised to use timeout exclusively for low-level calculations. It should be omitted from the input when performing high-level calculations.