F3 Structures and Frequencies

These SVWN5 results are somewhat fortuitous. Be careful not to overgeneralize from their agreement to experiment. We will see a different result in Exercise 6.7. Several other excerises will also include comparisons of DFT methods to Hartree-Fock theory, MP2 and other electron correlation methods. 

Exploring Chemistry with Electronic Structure Methods 

We ve alluded to the fact that more accurate calculations come only at the expense of greater computational cost. In this section, we d like to make that statement more concrete. We ll also look at memory requirements for jobs involving f and higher basi.s functions. 

The table on the next page indicates the relationship between problem size and resource requirements for various theoretical methods. Problem size is measured primarily as the total number of basis functions (N) involved in a calculation, which itself depends on both the system size and the basis set chosen some items depend also on the number of occupied and virtual (unoccupied) orbitals (O and V respectively), which again depend on both the molecular system and the basis set. The table lists both the formal, algorithmic dependence and the actual dependence as implemented in Gaussian (as of this writing), which may be somewhat better due to various computational techniques 

The table indicates how resource usage varies by problem size. For example, it indicates that for direct MP2 energy calculations, CPU requirements scale roughly as the fourth power of the number of basis functions if the number of electrons stays the same. Using the table with timings from previous jobs (using the same method and executed on the same computer system) should enable you to estimate how long a potential job will run. 

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