# 4.2.25. GUGACI¶

## 4.2.25.1. Description¶

The GUGACI program generates Multi Reference SDCI wavefunctions. The program is based on the Direct CI method [59], and with distict row table generated by GUGADRT (See program description for GUGADRT). The hole-particle symmetry based on GUGA is used in GUGACI [79][80]. If requested, GUGACI computes matrix elements of those one-electron properties for which it can find integrals in the ONEINT file. It also generates natural orbitals that can be fed into the property program to evaluate certain one electron properties. The natural orbitals are also useful for Iterated Natural Orbital (INO) calculations.

The GUGACI code is written by Yubin Wang, Zhenyi Wen, Zhenting Gan, Bingbing Suo and Yibo Lei (Institute of Modern Physics, Northwest University, China).

The program can calculate several eigenvectors simultaneously.

### 4.2.25.1.1. Orbital subspaces¶

The orbital space is divided into the following subspaces: Frozen, Inactive, Active, Secondary, and Deleted orbitals. Within each symmetry type, they follow this order.

• Frozen: Frozen orbitals are always doubly occupied, i.e., they are not correlated. Orbitals should be frozen already in the integral transformation step, program MOTRA, and need not be specified in the input to the MRCI program. If it’s specified, it will be ignored.

• Inactive: Inactive orbitals are doubly occupied in all reference configurations, but excitations out of this orbital space are allowed in the final CI wavefunction, i.e., they are correlated but have two electrons in all reference configurations.

• Active: Active orbitals are those which may have different occupation in different reference configurations.

• Secondary: This subspace is empty in all reference configurations, but may be populated with up to two electrons in the excited configurations. This subspace is not explicitly specified, but consists of the orbitals which are left over when other spaces are accounted for.

• Deleted: This orbital subspace does not participate in the CI wavefunction at all. Typically the 3s,4p,… components of 3d,4f,…, or orbitals that essentially describe core correlation, are deleted. Similar to freezing, deleting should be done in MOTRA, which is more efficient, and do not need to be specified in the MRCI program.

Since ordinarily the frozen and deleted orbitals were handled by MOTRA and the subdivision into inactive and active orbitals were defined in GUGADRT, program GUGACI will neglect them.

## 4.2.25.2. Dependencies¶

The program needs the distict row table generated by the program GUGADRT and transformed one- and two-electron integrals generated by the program MOTRA.

## 4.2.25.3. Files¶

### 4.2.25.3.1. Input files¶

TRAINT*

Transformed two-electron integrals from MOTRA.

TRAONE

Transformed one-electron integrals from MOTRA.

ONEINT

One-electron property integrals from SEWARD.

MRCIVECT

Used for input only in restart case.

### 4.2.25.3.2. Output files¶

CIORBnn

One or more sets of natural orbitals, one for each CI root, where nn stands for 01,02, etc.

CIVECT

CI vector, for later restart.

Note that these file names are the FORTRAN file names used by the program, so they have to be mapped to the actual file names. This is usually done automatically in the Molcas system. However, in the case of several different numbered files

### 4.2.25.3.3. Local files¶

FTxxF01

MRCI produces a few scratch files that are not needed by any other program in Molcas. Presently, these are xx=14, 15, 16, 21, 25, 26, 27, and 30. The files are opened, used, closed and removed automatically. See source code for further information.

## 4.2.25.4. Input¶

This section describes the input to the GUGACI program in the Molcas program system, with the program name:

&MRCI


### 4.2.25.4.1. Keywords¶

The first four characters are decoded and the rest are ignored.

TITLe

The lines following this keyword are treated as title lines, until another keyword is encountered. A maximum of ten lines is allowed.

NRROots

Specifies the number of CI roots (states) to be simultaneously optimized. The default is 1. The value is read from the next line.

RESTart

Restart the calculation from a previous calculation. No additional input is required. The MRCIVECT file is required for restarted calculations.

THRPrint

Threshold for printout of the wavefunction. All configurations with a coefficient greater than this threshold are printed. The default is 0.05. The value is read from the line following the keyword.

CONvergence

Energy convergence threshold. The result is converged when the energy of all roots has been lowered less than this threshold in the last iteration. The default is 1.0d-8. The value is read from the line following the keyword.

PRINt

Print level of the program. Default is 5. The value is read from the line following the keyword.

MAXIterations

Maximum number of iterations. Default 20. The value is read from the line following the keyword. The maximum possible value is 200.

PRORbitals

Threshold for printing natural orbitals. Only orbitals with occupation number larger than this threshold appears in the printed output. The value is read from the line following the keyword. Default is 1.0d-5.

CPROperty

Request to calculate properties. Property integrals should be saved in file ONEINT.

### 4.2.25.4.2. Input example¶

&GUGACI
Title
Water molecule. 1S frozen in transformation.
Nrroots
1