Log to sys.stderr if the user doesn't specify a file.

README.md

@@ -215,9 +215,10 @@ where the columns specify the current step, the in vacuo energy and the total
 energy.
 
 General log messages are written to the file specified by the `--log-file` or
-`EMLE_LOG_FILE` options. (The default is `emle_log.txt`.) The log level can be
-adjusted by using the `--log-level` or `EMLE_LOG_LEVEL` options. For performance,
-the default log level is set to `ERROR`.
+`EMLE_LOG_FILE` options. (By default, no log file is used and diagnostic messages
+are written to `sys.stderr`.) The log level can be adjusted by using the
+`--log-level` or `EMLE_LOG_LEVEL` options. For performance, the default log level
+is set to `ERROR`.
 
 ```
 #     Step            E_vac (Eh)            E_tot (Eh)

emle/calculator.py

@@ -35,6 +35,7 @@
 import shlex as _shlex
 import shutil as _shutil
 import subprocess as _subprocess
+import sys as _sys
 import tempfile as _tempfile
 import yaml as _yaml
 
@@ -389,7 +390,7 @@ def __init__(
         energy_frequency=1,
         energy_file="emle_energy.txt",
         log_level="ERROR",
-        log_file="emle_log.txt",
+        log_file=None,
         save_settings=True,
     ):
         """Constructor.
@@ -521,9 +522,7 @@ def __init__(
 
         # Validate the log file.
 
-        if log_file is None:
-            log_file = "emle_log.txt"
-        else:
+        if log_file is not None:
             if not isinstance(log_file, str):
                 raise TypeError("'log_file' must be of type 'str'")
 
@@ -536,7 +535,9 @@ def __init__(
                     raise IOError(
                         f"Unable to create directory for log file: {log_file}"
                     )
-        self._log_file = _os.path.abspath(log_file)
+            self._log_file = _os.path.abspath(log_file)
+        else:
+            self._log_file = _sys.stderr
 
         # Update the logger.
         _logger.remove()