Run CCS/VTZ for [H] instead of more complex WF methods

[alongd]

Methods such as DLPNO-CCSD(T) which is frequenctly used for kinetics due to it's efficiency and relatively good accuracy cannot be applied for [H]. This method relies on electron pairs for defining correlation spaces, and the H radical has only one electron.
Therefore, reactions such as RH + H <=> R + H2 cannot be computed using this method. However, since the energy of H is relatively simple, it is "known" percisely or can be computed using another Coupled-Cluster method without the DLPNO (Domain-based Local Pair Natural Orbital) assumption. This PR runs a simple and quick CC job for H if a DLPNO method was requested for it.

[OfirSimhi1612]

LGTM!

[Copilot]

Pull Request Overview

This PR implements automatic fallback to CCSD/VTZ for hydrogen atoms when DLPNO methods are requested. The motivation is that DLPNO-CCSD(T) methods, while efficient for larger molecules, cannot handle the hydrogen radical [H] which has only one electron and lacks the electron pairs required for DLPNO correlation spaces.

• Adds logic to detect hydrogen atoms and automatically use CCSD/VTZ instead of DLPNO methods
• Improves error messages and code robustness with minor fixes
• Removes unreachable code in SSH job status checking

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
arc/scheduler.py	Implements hydrogen detection logic and DLPNO method fallback to CCSD/VTZ
arc/species/species.py	Adds atom count check in isomorphism comparison and improves error message
arc/main.py	Improves error message clarity for invalid reaction inputs
arc/job/ssh.py	Removes unreachable code after return statement

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[Copilot]

The retry logic uses a magic number (5 retries, 10 second sleep). Consider extracting these as named constants or configuration parameters to improve maintainability.

Suggested change

	i = 5
	while i and not os.path.isfile(job.local_path_to_output_file):
	i -= 1
	time.sleep(10)
	i = RETRY_COUNT
	while i and not os.path.isfile(job.local_path_to_output_file):
	i -= 1
	time.sleep(RETRY_SLEEP_SECONDS)

[Copilot]

Pull Request Overview

This PR implements a fallback mechanism for computing hydrogen radical ([H]) energies when DLPNO-based methods are requested. DLPNO-CCSD(T) methods cannot be applied to single-electron systems like hydrogen radicals, so the code now automatically switches to a simpler CCSD/VTZ method for these cases.

• Adds hydrogen radical detection and method switching in single-point energy calculations
• Includes minor code quality improvements (error message clarification, defensive programming)
• Removes unreachable code in SSH job status checking

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
arc/scheduler.py	Implements H radical detection and automatic method switching from DLPNO to CCSD/VTZ, adds retry logic for output file checking
arc/species/species.py	Adds atom count check for isomorphism comparison optimization
arc/main.py	Improves error message clarity for invalid reaction input
arc/job/ssh.py	Removes unreachable code after return statement

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[OfirSimhi1612]

LGTM