def get_evac():
    """Get mean vacuum energy, if it has been calculated"""
    from pathlib import Path
    from asr.core import read_json
    evac = None
    if Path('results-asr.gs.json').is_file():
        results = read_json('results-asr.gs.json')
        if 'vacuumlevels' in results.keys():
            evac = results['vacuumlevels']['evacmean']
    return evac
