The U.S. Energy Department's chief of staff has resigned after an internal investigation conclude that her efforts to get DOE to hire her stepson "appear to violate the federal laws against nepotism."

Energy Secretary Charles W. Duncan Jr. yesterday announced the resignation of Deanne C. Siemer and said she had made "an error in judgement" by trying to arrange for her stepson's employment and then by authorizing a government-paid trip that the stepson made to Alaska and Canada last spring.

Duncan praised Siemer's "intelligence, performance and dedication to duty" as one of his top advisers on policy issues.

But Duncan said:

"I don't want my top people to use influence to get special favors."

The DOE's Office of Inspector General issued a 55-page report on the incident yesterday.

The report said "Siemer became insensitive to the distinction between maintaining the public trust and advancing the personal interests of her family." It recommended that Duncan take "appropriate disciplinary action."

Duncan wrote in answer to the report that Siemer already had asked to leave the department to return to private life, a request that he granted before the events involving her stepson took place. Therefore, he wrote, there was no need for diseciplinary action.

Duncan said Siemer remains a DOE employe working on a project but will be absent from the department for some time because of an illness in her family. Then, sometime this fall, she will leave DOE, Duncan said.

Duncan announced yesterday that Siemer's replacement as chief of staff is Douglas G. Robinson, who has been the deputy chief of the DOE's powerful regulatory arm, the Economic Regulatory Administration.

"Prior to that, Robinson was a member of the issues staff of the Carter-Mondale campaign in Atlanta, a job he left another high U.S. energy post to take.

Siemer issued a 29-page written document yesterday denying that she had violated any regulation or law. But she added that, "It was a substantial error of judgement to believe that I could effectively wall myself off from the hiring of my stepson by a DOE component and that after [his] hire I could deal with him like any other government employe."

Siemer said she will reimburse the DOE $807.23 that the government paid for her stepson's airfare to Alaska and Canada, and will also reimburse the department for $937.80 in expenses that he has claimed.

Siemer's statement charged that the DOE inspector general's report was biased and represented a conflict of interest and an "abuse of power" because she had at one time tried to "remove the inspector general for incompetence."

The inspector general, J. Kenneth Mansfield, had no comment on this charge yesterday but his deputy, Thomas S. Williamson, said, "The investigation was conducted in an entirely objective and open-minded fashion."

Before being brought by Duncan to the DOE in the summer of 1979, Siemer was the general counsel of the U.S. Department of Defense, a presidential appointment. The power of her office and her strong personality led some critics to refer to her as a "dragon lady."

Duncan then was deputy secretary of defense, a post he left when President Carter made him energy secretary Aug. 24, 1979.

The inspector general's report, based on a two-month investigation, alleges that Siemer brought pressure to bear to get a DOE summer intern job for her stepson, Jonathan Willens, a college student.

Willens is the son of Howard P. Willens, an attorney in the Washington law firm of Wilmer & Pickering -- the same firm of which his wife, Deanne Siemer, was a member before going to the Defense Department in 1977.

No one at any time has alleged that the Willens youth did anything wrong. He was described by officials as bright and hard-working.

The inspector general's report describes how, "at the insistence of Deanne Siemer," DOE officials scrambled last spring to try to find an intern job for Jonathan Willens. The report said the effort failed because there was no slot available.

"Ms. Siemer was able to trigger an extraordinary, although ultimately unsucessful, mobilization of DOE staff in the effort to hire Jonathan Willens," the report said.

Then, the report said, Siemer "inquired within the department" about having her stepson work for a DOE contractor. Other DOE officials, "acting on their own initiative rather than at the specific request of Ms. Siemer," then assisted the young man in finding such a . . . job, the report said.

After that, the Alaska-Canada trip was "arranged at the direction of Ms. Siemer," according to the report. On the trip -- from May 24 to June 4 -- young Willens did advance work for a trip that Duncan took later.

The IG report concluded that Siemer's alleged actions in recommending summer employment at DOE for young Willens "appear to violate the federal law against nepotism . . . In addition, those actions and her arranging her stepson's trip to Alaska appear to have violated several provisions of DOE's Conduct of Employees regulations that limit or prohibit preferential treatment of members of the public by DOE officials."

The report quotes Siemer as stating to investigators that, "I made no efforts to obtain employment, and I made no efforts to have him [Jonathan Willens] hired."