Official Resources
- Homepage: https://github.com/piecuch-group/ccpy
- Documentation: In repository
- Source Repository: https://github.com/piecuch-group/ccpy
- License: GNU General Public License v3.0
Overview
ccpy is a Python-based coupled-cluster package developed by the Piecuch group at Michigan State University. It implements a variety of ground and excited-state coupled-cluster methods using a hybrid Python-Fortran approach for computational efficiency.
Scientific domain: Coupled-cluster theory, excited states, post-HF methods
Target user community: Researchers applying advanced coupled-cluster methods
Theoretical Methods
- CCD, CCSD, CCSDT, CCSDTQ
- CCSD(T) and other perturbative triples
- Completely Renormalized CC (CR-CC)
- Equation-of-Motion CC (EOM-CC)
- Left eigenstate CC
- Active-space CC methods
- Similarity-transformed EOM
Capabilities (CRITICAL)
- Ground-state coupled cluster
- Excited-state EOM-CC
- Left eigenstates for properties
- Perturbative corrections
- CR-CC methods (CR-CC(2,3))
- Active-space extensions
- Hybrid Python/Fortran
- Efficient contractions
Key Strengths
Method Coverage:
- Multiple CC approximations
- Perturbative corrections
- Renormalized methods
- Excited states
Piecuch Methods:
- CR-CC(2,3)
- DEA/DIP-EOMCC
- Active-space variants
- Size-extensive corrections
Implementation:
- Python front-end
- Fortran performance
- Modular design
- Extensibility
Research Focus:
- Method development
- New approximations
- Benchmark calculations
Inputs & Outputs
-
Input formats:
- Molecular integrals
- Python scripts
-
Output data types:
- Energies
- Amplitudes
- Excitation energies
- Transition properties
Interfaces & Ecosystem
- PySCF: Integral interface
- NumPy: Array operations
- Fortran: Computation kernels
Advanced Features
CR-CC Methods:
- Non-iterative corrections
- Size-extensivity
- Multi-reference character
- Bond breaking
EOM-CC Variants:
- EOM-CCSD
- EOM-CCSDT
- IP/EA/EE variants
- Transition moments
Active Space:
- CCSDt
- CC(t;3)
- Reduced scaling
- Large systems
Performance Characteristics
- Speed: Fortran-accelerated
- Accuracy: High-level methods
- System size: Medium molecules
- Parallelization: OpenMP
Computational Cost
- CCSD(T): O(N^7) perturbative
- EOM-CCSD: O(N^6) per state
- CR-CC: Additional corrections
- Typical: Moderate molecules
Limitations & Known Constraints
- Documentation: Research-focused
- Large systems: Standard CC limitations
- User interface: Requires expertise
- Community: Research group centered
Comparison with Other Codes
- vs CFOUR: Both high-level CC; different methods
- vs MRCC: Both advanced CC
- vs ccq: ccpy more methods, Piecuch focus
- Unique strength: CR-CC, Piecuch group methods
Application Areas
Excited States:
- EOM-CC calculations
- Transition properties
- Multi-state problems
Strong Correlation:
- CR-CC for bond breaking
- Active-space methods
- Challenging systems
Benchmarks:
- Reference calculations
- Method validation
- New developments
Verification & Sources
Primary sources:
- GitHub: https://github.com/piecuch-group/ccpy
- Piecuch group publications
- Michigan State University
Confidence: VERIFIED
- Source code: OPEN (GitHub, GPL v3)
- Academic group: Piecuch group
- Active development: Yes