Specifies that the geometry is to be taken from the checkpoint file and that modifications will be made to it. A total of two input sections will be read: the first contains the charge and multiplicity, and the second contains alterations to the retrieved geometry. Note that in Gaussian 03, Modi is the shortest valid abbreviation for this keyword.
The first part of the Gaussian 16 output file states in considerable detail the contents of the license agreement. This should be taken seriously. Gaussian 16 is no public domain software!!
- Read Requests that the initial guess be read from the checkpoint file (Guess=Read is often specified along with Geom=Checkpoint). This option may be combined with Alter, in which case the orbitals are read from the checkpoint file, projected onto the current basis set, and then the specified alterations are made. Checkpoint is a synonym for Read.
- Read checkpoint file: Retrieves the initial guess from the checkpoint file (Guess=Read). Read checkpoint; otherwise generate: Check checkpoint file for the initial guess; generate if not present (Guess=TCheck). Read input checkpoint file: Read the guess from the checkpoint file whose name is specified in the input stream (Guess=Input).
- The Results=Summary menu item displays summary data about the results of the Gaussian calculation (available when a Gaussian log file or checkpoint file is opened). It is displayed in Figure 73. Summary of a Gaussian Calculation This window summarizes the results of a B3LYP/6-31G(d) frequency calculation.
Actual program output specific to a certain calculation starts with a statement of the program version (here Gaussian 16), program revision (here A.03), and the current date. Subsequently the keywords used in the input file are repeated together with other general settings such as the amount of main memory available for the calculations (here 16000MB), and the location of a binary checkpoint file for storage of important results (here /scr1/zipse/17079/watdim01.chk). The calculation performed here is the B3LYP/6-31+G(d,p) geometry optimization of the water dimer, complemented by the D3 dispersion correction.
The keywords are transformed by Gaussian into a sequence of subroutine calls termed 'links'. The links are given together with the corresponding options set for each link in a proprietary format. Provided that the '#P' option is used in the input file, Gaussian prints out elapsed CPU times after leaving a link.
In link L101 the program reads in or retrieves from the checkpoint file the structure of the system together with other parameters and prints the structure (in a slightly modified format) together with overall charge and spin multiplicity and the comments supplied in the input file. It is good practice to include the name of the input file in the comments of the job. The system chosen here is the water dimer in its electronic ground state.
Link L103 initiallizes the Berny geometry optimization algorithm named after its creator Bernhard Schlegel. This includes the generation of second derivative estimates for the optimization variables.
Link L202 determines, among others, the symmetry of the system, decides on the symmetry properties that will be used in the actual quantum mechanical calculations and rotates the molecule such that the center of mass is located in the origin of the cartesian coordinate system, the principal axis (so it exists) points along the z-axis, and the principal plane of symmetry (so it exists) is located in the yz-plane. The resulting orientation is printed as 'Standard orientation', which serves as the reference description for all information regarding the wavefunction and first and second derivatives of the energy with respect to structural parameters.
Link L301 loads all components necessary for the actual quantum mechanical part of the calculation. In addition, it also lists the nuclear repulsion energies as well as the London dispersion energies calculated accoring to the D3 formula (both in Hartree).
Link L302 calculates a number of integrals necessary for the subsequent SCF (energy) calculation.
Read Checkpoint File Gaussian Form
Before the actual energy calculation is performed, a guess for the wavefunction is obtained using either the Hueckel, the INDO, or the Harris functional method. Alternatively, a guess can also be read from the checkpoint or the input file.
Calculation of the B3LYP-D3/6-31+G(d,p) energy of the system is done in link 502. Some parameters such as the currently selected convergence criteria are listed first. The final SCF energy given as E(RB3LYP) = -152.878894550 is the energy of the system with respect to its nuclei and electrons at infinite separation. The energy is given in atomic units (Hartree).
Selected information on the optimized Kohn-Sham orbitals connected to the B3LYP energy calculation is printed along with a Mulliken population analysis in link 601. For all MOs of the system the program lists the irreducible representations (A' or A' for a Cs symmetric system), the MO energies (in Hartree), and the overal state symmetry of the system.
For each geometry optimization step, the Berny geometry optimization algorithm requires the calculation of the first derivatives of the energy with respect to all structural coordinates together with an estimate for the respective second derivatives. All of this is done in the L70x links.
The program then jumps back to the actual geometry optimizer in link L103, which lists the actual gradients for all optimization parameters in the '-DE/DX' column together with structural parameters with a reduced energy gradient. The optimization variable 'r2', for example, is predicted to have a slightly longer value listed in the 'New X' column as 1.83912. This value is given in atomic units, which for distance parameters equates to the Bohr unit (or 0.529177209 Angstroms). A distance of 1.83912 Bohr thus equates to 0.973218 Angstroms. A the end of this ouptut block we find a short summary of the current status of the geometry optimization, listing largest and average (that is room mean square, RMS) gradients for the system under study as well as maximum and RMS displacements of the structural coordinates.
The program then loops as many times as needed through links L202, L502, and L70x, and L103 to search for a geometry with lower energy gradients (output not shown here). Eventually, all four convergence criteria listed by link L103 are fulfilled and the geometry optimization is terminated.
Read Checkpoint File Gaussian Online
For the last (optimized) geometry the program then lists wavefunction characteristics and population analysis data before finishing off in an orderly manner with link L9999 by printing a compact archive entry. This latter block of text includes the optimized geometry, the symmetry of the system (here PG=CS), and the electronic energy (here stated as HF=-152.8788946).