The basis option allows certain standard Gaussian atomic-orbital basis sets to be easily requested.  

The value of the option must be a string.  Note that hyphens can be included in the basis set names.  

Mixed basis sets in which a different basis set is requested for each atom type can be specified by a string with the following format: "AtomicSymbol1:BasisSet1;AtomicSymbol2:BasisSet2;AtomicSymbol3:BasisSet3"  Note that in a mixed basis set the symbols and basis sets are separated by a colon ":" while the atomic-symbol/basis-set pairs are separated by a semicolon ";".  When using a mixed basis set, a basis set must be given for each type of atom in the molecule.

Effective core potentials (ECP), which replace the core electrons with an effective potential, are useful for treating heavier atoms because they decrease the computational cost while including some relativistic effects.  ECPs are automatically employed in molecular calculations in the Quantum Chemistry Package for the following ECP-designed basis sets:  "bfd_vdz", "bfd_vtz", "bfd_vqz", "bfd_v5z", "cc-pvdz-pp", "cc-pvtz-pp", "cc-pvqz-pp", "cc-pv5z-pp", "crenbl", "crenbs", "def2-svp", "def2-svpd",
"def2-tzvp", "def2-tzvpd", "def2-tzvpp", "def2-tzvppd", "def2-qzvp", "def2-qzvpd", "def2-qzvpp", "def2-qzvppd", "lanl08",
"lanl2dz", "lanl2tz", "sbkjc", "stuttgart_dz", and "stuttgart_rsc".

The basis set can be augmented with ghost atoms.  See Ghost Atoms.

The SearchBasisSets command can be used to search for a specific basis set or family of basis sets.

The complete set of possible basis sets are as follows:
     321++g*      
     321++g       
     321g*        
     321g         
     431g         
     6311++g**    
     6311++g*     
     6311++g      
     6311+g**     
     6311+g*      
     6311+g       
     6311g**      
     6311g*       
     6311g        
     631++g**     
     631++g*      
     631++g       
     631+g**      
     631+g*       
     631+g        
     631g**       
     631g*       
     631g         
     adzp         
     ahlrichs     
     ano          
     anoroosdz    
     anoroostz   
     aqzp         
     atzp         
     augccpv5z    
     augccpv5zdk   
     augccpv5zdkh  
     augccpv5zjkfit   
     augccpv5zmp2fit  
     augccpv5zoptri   
     augccpv5zpp  
     augccpvdz    
     augccpvdzdk   
     augccpvdzdkh  
     augccpvdzjkfit   
     augccpvdzoptri   
     augccpvdzpjkfit  
     augccpvdzpp  
     augccpvdzpri     
     augccpvdzri      
     augccpvqz    
     augccpvqzdk   
     augccpvqzdk3  
     augccpvqzdkh  
     augccpvqzjkfit   
     augccpvqzmp2fit  
     augccpvqzoptri   
     augccpvqzpp  
     augccpvqzri      
     augccpvtz    
     augccpvtzdk   
     augccpvtzdk3  
     augccpvtzdkh  
     augccpvtzjkfit   
     augccpvtzoptri   
     augccpvtzpp  
     augccpvtzri      
     augccpwcvqzdk   
     augccpwcvqzdk3  
     augccpwcvtzdk   
     augccpwcvtzdk3  
     augccpwcvtzmp2fit  
     bfd          
     bfdpp        
     bfdv5z       
     bfdvdz       
     bfdvqz       
     bfdvtz       
     ccpcv6z     
     ccpcvdz      
     ccpcvdzf12optri  
     ccpcvqz       
     ccpcvqzf12optri  
     ccpcvtz      
     ccpcvtzf12optri  
     ccpv5z       
     ccpv5zdk     
     ccpv5zdkh    
     ccpv5zf12        
     ccpv5zf12rev2    
     ccpv5zjkfit   
     ccpv5zmp2fit  
     ccpv5zpp     
     ccpv5zri      
     ccpvdz       
     ccpvdzdk     
     ccpvdzdkh    
     ccpvdzf12nz      
     ccpvdzf12optri   
     ccpvdzf12rev2    
     ccpvdzfit    
     ccpvdzjkfit   
     ccpvdzpp     
     ccpvdzppnr   
     ccpvdzri      
     ccpvqz       
     ccpvqzdk     
     ccpvqzdk3     
     ccpvqzdkh    
     ccpvqzf12nz      
     ccpvqzf12optri   
     ccpvqzf12rev2        
     ccpvqzjkfit   
     ccpvqzmp2fit  
     ccpvqzpp     
     ccpvqzri      
     ccpvtz       
     ccpvtzdk     
     ccpvtzdk3     
     ccpvtzdkh    
     ccpvtzf12nz      
     ccpvtzf12optri   
     ccpvtzf12rev2    
     ccpvtzfit    
     ccpvtzjkfit   
     ccpvtzpp    
     ccpvtzppnr   
     ccpvtzri      
     ccpwcv5z     
     ccpwcv5zpp   
     ccpwcvdz     
     ccpwcvdzdk   
     ccpwcvdzpp   
     ccpwcvqz     
     ccpwcvqzdk   
     ccpwcvqzdk3  
     ccpwcvqzpp   
     ccpwcvtz     
     ccpwcvtzdk   
     ccpwcvtzdk3  
     ccpwcvtzmp2fit  
     ccpwcvtzpp   
     crenbl       
     crenbs       
     def2qzvp     
     def2qzvpd    
     def2qzvpjfit     
     def2qzvpjkfit    
     def2qzvpp    
     def2qzvppd   
     def2qzvppdri     
     def2qzvppjfit    
     def2qzvppjkfit   
     def2qzvppri      
     def2qzvpri       
     def2svp      
     def2svpd     
     def2svpdri       
     def2svpjfit      
     def2svpjkfit     
     def2svpri        
     def2tzvp     
     def2tzvpd    
     def2tzvpdri      
     def2tzvpjfit     
     def2tzvpjkfit    
     def2tzvpp    
     def2tzvppd   
     def2tzvppdri     
     def2tzvppjfit    
     def2tzvppjkfit   
     def2tzvppri      
     def2tzvpri       
     demon        
     dgaussa1cfit  
     dgaussa1xfit  
     dgaussa2cfit  
     dgaussa2xfit  
     dyalldz      
     dyallqz      
     dyalltz      
     dz           
     dzp          
     dzpdk        
     dzpdkh       
     dzpdunning   
     dzvp         
     dzvp2        
     faegredz     
     heavyaugccpvdzjkfit   
     heavyaugccpvtzjkfit   
     iglo         
     iglo3        
     lanl08       
     lanl2dz      
     lanl2tz      
     minao        
     qzp          
     qzpdk        
     qzpdkh       
     roosdz       
     roostz       
     sbkjc        
     sto3g        
     sto6g        
     stuttgart    
     stuttgartdz  
     stuttgartrlc  
     stuttgartrsc  
     tzp          
     tzpdk        
     tzpdkh       
     tzv          
     weigend      
     weigend+etb  

$\mathrm{with}\left(\mathrm{QuantumChemistry}\right)\:$

$$\begin{gathered} \mathrm{O2} ≔ \left[ \left[''O''\, 0\, 0\, -0.6\right]\, \left[''O''\, 0\, 0\, 0.6\right] \right]\; \\ \mathrm{ans} ≔ \mathrm{HartreeFock}\left(\mathrm{O2} \, \mathrm{basis}\=''cc-pvdz''\right)\; \end{gathered}$$

$\mathrm{O2}≔\left[\left[''O''\,0\,0\,\mathrm{-0.60000000}\right]\,\left[''O''\,0\,0\,0.60000000\right]\right]$

$$\begin{gathered} \mathrm{gold} ≔ \left[ \left[''Au''\, 0\, 0\, 0\right] \right]\; \\ \mathrm{ans} ≔ \mathrm{Parametric2RDM}\left(\mathrm{gold} \, \mathrm{basis}\=''lanl2dz''\right)\; \end{gathered}$$

$\mathrm{gold}≔\left[\left[''Au''\,0\,0\,0\right]\right]$

$$\begin{gathered} \mathrm{goldH} ≔ \left[ \left[''Au''\, 0\, 0\, 0\right]\,\left[''H''\,0\,0\,2\right] \right]\; \\ \mathrm{ans} ≔ \mathrm{Parametric2RDM}\left(\mathrm{goldH} \, \mathrm{basis}\=''Au:lanl2dz;H:dz''\right)\; \end{gathered}$$

$\mathrm{goldH}≔\left[\left[''Au''\,0\,0\,0\right]\,\left[''H''\,0\,0\,2\right]\right]$